Preparing to Train and Lucifer's Sister

A 3-year-old boy was sleeping on a bed that was 3 times his size.

The boy has short purple hair with blue eyes and wearing a green one-suit pajama.

This boy is Kei Astaroth, our protagonist and the successor of the unknown Dragon God.

It was 3 years ago that he was given birth by Belona Astaroth, his mother.

At the same time as his birth, however, a girl with red hair and blue eyes were born.

When both of them were born, the world was in gaiety.

Rainbows can be seen at almost every part of Earth. It was like the world was delighted for their birth, even though both of them were born as a devil.

Kei slowly opened his eyes.

He woke up feeling strangely rejuvenated. He didn't quite remember what he did last night, but something had happened when he was asleep.

Dragons have a core inside of them. This core act as something similar to a mana generator.

Every dragon has them, whether it was only a newborn or an old dragon, a normal dragon or a Dragon God.

Humans can also have dragon cores. Those that have one generally have powers similar to a dragon.

High magic resistance, high physical resistance, etc.

Sometimes, a person that was unaware that they have a dragon core usually a person that was given one either before or after birth.

And the dragon core usually in a sleep state.

To wake it up, one would need high will power and suitable strength.

But this doesn't work for Kei.

Last night, he woke his core unconsciously.

The moment when he finally aware of his surroundings is the moment where his core woke up from its previous state.

When his core started waking up, there was a harmless blast of mana coming out of Kei's body.

People in his 3 KM range felt very invigorated.

The closer a person to Kei's range, the more apparent that feeling will be.

And this was the case for Belona, Devon, and Astaroth's maids and butlers.

Some of the maids that were exhausted suddenly re-energized, the same could be said for the butlers.

His parents were also exhausted. They were just doing a married couple's 'business' and wanted to rest when this happened.

Without a doubt, they were back doing their 'business' as well.

His core waking up did not only affect his surroundings, but also Kei himself.

When he woke up, his body was strengthened, his brainpower was also increased to the point of him having an eidetic memory, his perception was sharper than ever.

That's why he felt so happy and refreshed when he woke up.

Kei's first order of business was to take a bath, and so he did.

He opened his pajama to take a bath.

He was feeling giddy. Because his brother, Ajuka Beelzebub, asked him to come with him to meet his brother's friend.

The strongest Maou, Sirzech Lucifer.

Ajuka also said that Sirzech will bring his sister with him today.

'I will finally get to meet Rias.' he thought in his mind while showering.

Rias Gremory, the heiress of the Gremory clan after his brother took the Lucifer title.

She has a long crimson hair that reaches her thighs and blue eyes. That is her style when she grew up to become a Kuoh student.

She has the Power of Destruction that she inherited from her mother, Venelana Gremory.

It was unknown what kind of special ability the Gremory has either than their powerful demonic powers and superior control over it. But because her mother was from Bael clan, the clan is known for their Power of Destruction, Rias and her brother possessed the Power of Destruction.

When he reminiscing on his past life where he enjoys watching anime. He suddenly remembered that Rias was supposed to be engaged to Riser Phenex.

Riser Phenex hailed from the Phenex clan.

In the early days of the original High School DxD. Riser was known as an arrogant flaming chicken with a holier-than-thou attitude.

Why chicken? Because the Phenex clan has similar powers as the legendary phoenix bird.

And chicken is a bird.

Phenex clan's wings are not bat-like wings like other devils. Instead, they spurt wings of fire to achieve flight.

"I need to train myself right now." Kei's said with motivation.

After he took a shower, he went to the dining room.

Usually, in the dining room, 3 maids and a butler stand by.

Now, however, there was only 1 maid that was standing beside his supposed dining chair and a butler.

The maid's job is to prepare today's meal on the plate while the butler is to tell the cook if the dine participants have any complaints about certain dishes and also if the participants want something that's not in the dining table.

"Where's mother and father?" Kei asked the maid.

"Unfortunately, master and mistress cannot dine with young master this morning." said the maid.

The maid is scarily resembled Rem from Re: Zero. The maid even has the same name.

The only difference between this Rem and the Rem from Re: Zero is that this Rem is a devil instead of an Oni.

Their past was also different.

This Rem was born out of wedlock. Fortunately for her and her mother, she was born in Astaroth's territory, or rather, inside the Astaroth's mansion.

She was already a maid from birth. Though this doesn't mean that she has a cruel past.

Instead, she was loved by all of the occupants of the Astaroth mansion, be it the maids, butlers, guards, and even the owner of the mansion loved her dearly.

On a side note, there is another version of Ram.

"Well, what's the breakfast menu for today?"

"We have a blue cheese quiche, waffle topped with chocolate and blueberry sauce, and english breakfast tea."

"I'll take the quiche and the tea, please. And please prepare a cheese souffle after I'm done eating."

"Yes, young master." Rem said while gesturing hand signs towards the butler.

The butler then walked out of the room to tell the chef.

The butler is called Sebastian Michaelis, and he resembles a butler from Kuroshitsuji that has the same name.

Sebastian has black hair with slit pupils similar to a cat.

Sebastian is a devil as he has another form that he usually takes for combat, which is currently unknown.

After he was done eating his breakfast and the cheese souffle, he immediately went back to his room.

The reason for this was because he wanted to try entering his dimension and train there for a time.

He wanted to be strong fast.

He knew he could since he's now a devil and was given a strong bloodline.

After he went inside his bedroom. He locked it and prepared himself to enter it.

He sat down in a lotus position and started meditating.

It was unknown how long that Kei managed to enter his dimension, but when he finally did, he can't move his body.

'What is this? Why can't I move my body?' Kei panicked

He won't panic if he can open his eyes.

Kei tried his best to open his eyes, but he can't. No matter how much time it took for him to open his eyes, it was like something invisible locked his muscles in place.

The only silver lining to this was that he can still breathe.

If he managed to open his eyes, he will be confused as to why he can breathe as he was in a very dark place similar to the void.

In the vast ocean of darkness, there's nothing that can be seen.

No lights, no trees, nothing.

It was not until 5 minutes had passed that he finally managed to calm himself down.

Though he cannot open his eyes, Kei can still feel his entire body strengthened to the point that if he tries to hit a rock with his current strength, the rock will break into pieces.

No one knows if this is thanks to his new eidetic memory or because he liked anime so much, but he remembered something back when he watched Dragon Ball Super.

Goku and Vegeta 'sent' by an angel called Whis to a dimension. When they entered this dimension, they cannot move their bodies unless they control his Ki.

So he tried the same thing.

His first hurdle was to feel his mana. It didn't take a very long time for him because of how huge his mana reserves are compared to someone of his age.

He can feel that he emits mana unconsciously, so he tried to control them.

But before that, he needs to change the time dilation because he can tell that this process will be a very long one.

And he made the right choice.

If he managed to do simple things like walking, running, etc, he essentially became one of the beings that managed to perfectly control their mana to the point where people will perceive Kei as someone that has no mana.

Even the God of the Bible from High School DxD, which is his new home world, can't match his control of mana.

He changed the time dilation into one second outside equals one year inside.

------------------------

1 month into his training, he can finally open his eyes with difficulty.

And only now that he realized he was in a similar place like the void.

He didn't like that one bit, because it made him remember the feeling that he felt when he was run over by that stupid truck.

So he 'will' his dimension to have an environment similar to a flower garden when he met his Goddess, Noire.

The scene instantly changed into one.

Flowers of all kinds can be found here. The scent from the flowers is so relaxing that Kei almost forgot his reason to go to his dimension.

He continued his control over mana.

--------------------------

6 months into his training, he can feel something keeps generating mana inside him.

His second hurdle is to try and control the 'thing' that kept generating mana.

He still didn't notice that the 'thing' he mentioned was a dragon core, his dragon core.

It took him one and a half years to just ignore his core because it cannot be 'switched off', at least for now.

Because of his core, it took him almost 3 years to finally be free from his restraints.

If his core is a 'normal' core then he won't even need 3 whole years.

"Fwuh, finally free," said Kei.

He was quite sick of being inside the dimension already, so after he said that, he went back to the outside world.

It was only 3 seconds outside, so almost no one noticed him gone, except the one that kept watching him.

She has long black hair with grey eyes. Her ears are pointy, not like what normal humans have.

She has a petite figure, combined with her clothing which was a black gothic lolita fashion, made people that never saw her think of her as a little girl.

When in reality, she's THE Ouroboros Dragon, Ophis.

She started watching him when she felt a burst of mana came from Kei's body.

Ophis knew that every dragon has a core, but this youngling has a different core with properties much purer than her own.

What she meant was that his core generates mana that's so pure that it was supposed to not exist in this world.

Ophis even 'joined' him in his bath. So she has been following Kei everywhere.

When he went to his dimension, all Ophis saw was that he suddenly disappeared from the Underworld before appearing 3 seconds later with power several times higher than before.

Why Ophis can sense his power? It was because he intentionally 'flare-up' his power for a second before adjusting it back to his 'normal' state where he unconsciously emits mana, but it was Kei consciously emits it this time to avoid suspicion.

Kei flared up his power so skillfully that only Ophis or someone inside the room can sense it.

'Interesting youngling. Perhaps he can bring back the silence that I once had.' thought Ophis before she disappeared from the Underworld.

Kei can sense Ophis instantly after he came back from his dimension.

Her mana was so massive that he knew why she was called the Dragon of Infinity by people.

He wanted to greet her but she's already gone and can't be sense anymore.

So he decides to greet her at a later date because it was nearing the time for him to go with his brother.

"I need to prepare my clothes." Kei then wear clothing that screams 'noble'

He didn't want to wear that kind of clothing, because it was not his style.

But because he didn't want to embarrass his brother, he relented.

While waiting, he tried to sense mana in the underworld.

The mana in the underworld is so 'demonic' in nature. The demonic energy in the underworld can help him strengthen his body.

Since he didn't know how to conjure a spell, he decides to just increase his strength in the physical department to the point that he's the same as Saitama from One Punch Man, or at least the same as Yamamoto from Bleach.

One hour later and there was a knock from the door.

"Young master, Ajuka-sama is waiting for you downstairs," said Rem from behind the door.

"Yes, I'm coming right now."

When Kei opened the door, he saw not only Rem but also his brother.

"Onii-sama, I thought you were waiting for me downstairs?"

"That's the plan, but I think it'll be faster if we just teleport to the Gremory mansion from your room."

"Huh?"

"Well, let's go," said Ajuka.

Ajuka conjures his magic circle to teleport them to in front of the Gremory mansion.

The scene changes from a big door in front of them to a fountain.

"Onii-sama, why did you want to bring me here again?"

"I wanted to bring you to meet my best friend, Sirzechs Lucifer. And become friends with his sister, Rias Gremory."

"Okay, Onii-sama."

Both of them then walked to a person standing in front of a big brown door.

"Ajuka, it is nice to see you again." The person greets them.

"Likewise, Sirzechs." Ajuka returns his greeting

"So, this is your new little brother huh? He's very similar to your mother."

"This devil greets the demon king, Lucifer-sama," said Kei while he put his right hand to his chest, his left hand behind his back and bow down a little bit.

"Your brother is certainly very mature for his age."

"Hahaha. Well, I suppose we go inside?"

"Yeah, let's go inside." as Sirzechs said that, the door opened.

Kei's eyes are greeted with a line of maids from behind the door.

It was an amazing sight, considering he never even see that kind of thing in his parent's mansion.

Then, they went to the dining room.

Both Sirzech and Ajuka were conversing while they walked to the room. It was like they haven't seen each other for a long time, or it was just that their friendship is that 'tight'.

"Onii-sama!" a girl with crimson hair standing in front of the door to the dining room.

"Ah, Rias-tan. Why did you not go inside the room?" asked Sirzechs

"Well, I was curious to see your friend and his brother," said Rias while running to Sirzechs

"Are you Kei?" asked Rias.

"Yes, Rias-sama. I am Kei Astaroth, the brother of Ajuka Beelzebub." Kei didn't know if he was supposed to be very formal or not.

"Moo, that is too formal. Call me Rias" said Rias while pouting.

Rias's pouting face certainly deal damage not only Sirzechs heart but also Kei's.

'How can she be so cute' both Kei and Sirzechs thought at the same time.

They haven't noticed, but Rias is especially excited when she saw Kei. It was like she found something precious.

Because she's still a child, she didn't know anything about love. So she interpreted her feelings as found something similar to what her Onii-sama has, a friend.

"Okay, Rias-san."

"Moo..." she pouted the second time.

'If this keeps going, I'm going to die from the cuteness.' Kei and Sirzechs thought at the same time again.

"Umm… Okay, Rias," said Kei while he glances over to Sirzechs.

Kei saw Sirzechs face frowned when Kei said that. It was obvious that his sis-con mind was alarmed.

'Am I going to make an enemy out of my brother's friend?' thought Kei.

It was not his fault, but he can't help but fear Sirzechs a little bit.

"Now now, let's go to the dining room already," said Ajuka, definitely noticed Sirzechs dislike towards his little brother.

Ajuka can only sweat drop at that.

"Did you already eat, Kei?" Rias asked

"Yes, I did, Rias."

"Let's go to my room instead to play." Sirzechs heart began thumping fast.

If not for Kei is his friend's little brother, Sirzechs probably will obliterate Kei on the spot. Rias hate be damned.

"Is it okay?" asked Kei while looking at both his brother and Sirzechs.

Sirzechs mouth twitched before saying, "Yes, you can."

"Thanks, Onii-sama. Let's go, Kei." Rias said while grabbing Kei's hand and 'kidnapped' him to her bedroom.

Rias' bedroom looked like what a princess bedroom should.

The bed is a king-sized bed, similar to what Kei uses. There are dolls and other girl toys all over the floor, making what a seemingly elegant children's bedroom to a messy girl's room.

"Ah, please wait at the door." Rias then closed the door, leaving Kei to wait for her.

There were sounds of someone 'cleaning' a messy room that can be heard even from the outside.

After 10 minutes had passed, Kei was permitted to go in.

"Hehe, sorry for the mess earlier, Kei."

"It's okay, Rias. So, what are we going to play?" Since Kei's in a child's body with a young adult mentality, he didn't want to play children's games. But for Rias, he'll do anything to make her happy.

Kei didn't realize that when he finally met Rias in person, he essentially becomes a man that met his first love.

It seemed 'wrong', but his subconscious mind already made its decision. Kei was taken by surprise when Rias wanted to play chess instead of 'playing house'.

In his past life, Kei was a chess world champion. He can even beat what a seemingly impossible-to-beat AI to a pulp.

"Are you sure you wanted to play chess with me? I'm quite good at it," said Kei.

"Let's see if you can even beat me, the heiress of Gremory house."

3 hours later, the score was 9-1 with Kei actually needed to give her handicap and also letting her win the final round.

"Muuu.. you are quite good." said a pouting Rias.

"We can play chess again next time. It's almost time for me to go back to the house with my brother."

"Eehhh? Can you stay? Please?" Rias made a 'child that was on the verge of tears' look.

Kei almost gave in, but fortunately, or unfortunately, his brother and Sirzechs came to his 'safety'.

So Kei and Ajuka went back with Kei not forgetting to tell her that he will play with her again next week.

"So, how's Rias?" Ajuka asked

"She's alright, Onii-sama."

"Nothing else besides 'alright'?" Ajuka teased Kei.

"Well, she's surely beautiful," said Kei with a bashful expression on his face.

"Hahaha, I hope you can be more than friends with her." Since Kei's birth that day, he dreams of Astaroth's family union with the Gremory's. Even though both Sirzechs and Ajuka didn't use the clan's name anymore, they're still hailed from their respective clans.

Though being best friends with each other is good, it was a nice thing if they become related for real.

After they teleported at Astaroth's mansion, Ajuka said goodbye to Kei before went his own way.

'Entering the dimension is probably the best thing that I can do, right now. But first...'

Kei went inside his room and locked it before saying, "You can come out now."

Suddenly, Ophis appeared right in front of him.

She's intrigued by his ability to sense her, even though she hides her presence.

"It's good to finally meet you, Ouroboros Dragon, or should I say, Ophis," said Kei.

Kei was slightly afraid by Ophis. Her power is far beyond him, at least for now.

"Youngling, can you bring me silence?" asked Ophis.

Kei knew what she was referring to.

Ophis have once occupied the Dimensional Gap, a 'void' that connects the three worlds, Heaven, Earth, and Underworld.

Nothing can survive in this dimensional gap without any magical protection, aside from the Great Red and Ophis.

Back in the story, Ophis once occupies the dimensional gap before she leaves for reasons unknown before going back and noticed that the dimensional gap is now occupied by Great Red, the Dragon of Dreams.

"For now, I can't. Give me enough time then I can help you with whatever it is you are dealing with." Kei felt that if he trains hard inside his dimension, he can become an unstoppable force in this world.

Kei suddenly thought if he just needs to bring Ophis to his dimension. But he dismissed the idea before keeping it in the back of his head.

Kei currently didn't know how his dimension worked and needed to study it a bit longer before he can bring someone there. He didn't want to endanger Ophis because he was quite fond of her even back when she's just a 2D character from an anime.

Ophis just nodded her head before she 'disappeared'.

She's still in his room, waiting for him to get stronger before bringing him to the dimensional gap.

"Just for your info, you can come back when I finally get my evil pieces." Kei already has a plan to deal with Great Red. He needed someone from another world that has Dragon Slayer Magic. And he knows who he will bring along from that world.

After Kei said that, Ophis' presence disappeared completely from his room.

"Let's enter the dimension." Kei sat in a lotus position and meditate.

It didn't take long for him to finally enter his dimension once again and without any difficulty in doing everyday stuff whatsoever.

Since the time dilation was still the same, one second outside equals to one year inside, Kei didn't mess with it.

"Hmm… what should I do now? Strengthen my foundation before learning magic? Or try to learn magic on my own?"

Kei decides to strengthen his foundation.

He changed the gravity of the dimension to somewhere around 10 times normal gravity.

Though, "normal gravity", in this case, refers to the gravity on the Underworld, which is not much higher compared to Earth's.

He started to do stretching before he started running.

His training for a year is doing the same exercise as Saitama's physical training and went from there.

Since Kei is not a human anymore, he deliberately intensifies the training by doubling the physical training. From 10 KM run, 100 push-ups, 100 sit-ups, 100 squats, to 20 KM run and so on.

He didn't forget to bring his training clothes that consist of a jacket, a white shirt, black cotton pants, and green shoes.

Since he didn't know where to start his training, he decides to just do the physical exercise until it didn't have any significant effect anymore.

---------------------------

3 months in and he can't feel any difference anymore.

The first month was the most effective one. He can feel that everything about him except his mana reserves getting stronger and more powerful.

He didn't know what to do anymore. He was in thought before remembering his second 'wish', about him inheriting Yamamoto's Zanpakuto.

He summoned the sword into his hand. He didn't expect a broken and charred Katana in his hand.

The blade of the Katana was gone, the handle is almost gone too.

The once-proud Zanpakuto now only consists of its guard and half of its handle.

Kei inspects the Zanpakuto before sitting in a lotus position and put the Zanpakuto on his lap.

When Kei opened his eyes, he was greeted by a vast land that consists of mountains, canyons, and holes that splurt out pillars of fire.

"So, you finally came." the sound of an old man can be heard from behind Kei's back.

Kei immediately turned and saw nothing.

"It seems you have a good enough strength to wield me." Kei then turned to the source of the sound.

It was an old man with a bald head, red eyes, and a very long beard. He wore a black shinigami clothing with a captain haori. He has a scar on his head that looked like an 'x' right on top of his right eye.

"Yamamoto?"

"No, I'm not Yamamoto. I'm the Zanpakuto spirit that you're trying to call. I only use his form so we can converse better. Though you have enough strength, you do not have the skill to wield me." As 'Yamamoto' speak, he attacked Kei with just his fist.

Kei's upper body disappeared with the punch.

When Kei came back, he was coughing blood. It didn't take Kei a long time to understand that he needed to train his swordsmanship first before he can come back.

It also made Kei understand that the Zanpakuto spirit gave him a chance.

Now that Kei knew what he was supposed to train, he was thinking the best way to increase his skill in swordsmanship.

"What should I do?" Kei was confused.

He can ask his father, but his father will not abide by his wish because Kei is still a child.

The personal dimension sensed his confusion and decided to 'summon' a fragment of Genryusai Shigekuni Yamamoto.

Noire didn't realize it, but the personal dimension has its own will because it has been over-saturated with the energy from the void, which Noire called 'Origin'.

In Kei's eyes, it was like a sudden light came before it turns into a person that he knew AND also that he just met.

"You are?!"

"You knew of me, young Kei?" said the person

Kei was surprised that Yamamoto knew his name.

"How did you know my name?"

"I don't know. The last thing I knew that I died from Yhwach's attack and suddenly brought here by someone. I was also given a purpose and a basic understanding of the situation. Now, you are supposed to inherit my Zanpakuto, but you are thrashed by it and this 'someone' needed my help to train you. Is that correct?" Though it was like Yamamoto asking Kei a question, he asked the Dimension itself.

Kei was confused, it was like Yamamoto asked him a question, but at the same time, he didn't.

Yamamoto receives something similar to an 'OK' sign inside his head.

"Okay then. Young Kei, because you are supposed to inherit my Zanpakuto, don't you ever dare to get out of here before you reach a power similar to me in my peak."

Kei knew that his training with Yamamoto will be a hard one. So he changed the time dilation from one second equals one year inside to one second outside equals three years inside.

Really... what an OP dimension that he has.