How to make Rias Gremory likeable?

To begin let's study Rias personality.

[Rias is a very kind and compassionate optimist, especially to her servants and people close to her. She gets angry and violent whenever people insult her or her peerage, showing a fiery side to her. This also plays into the fact that she dislikes cruelty towards people.

Rias is competitive and hates to lose, especially after losing her first Rating Game against Riser as she strives to always win, whether it is in sports or Rating Games.

Due to being spoiled by her father Zeoticus and her brother Sirzechs from an early age, she has a somewhat impatient and selfish personality and because of her bad money spending habit, her mother Venelana and her sister-in-law's Grayfia often scold her for her frivolous spending, as her mother reconsidered the amount of money she is allowed as allowance.] (From wiki)

I'll say it now, I hate Rias.

I understand that she was a spoiled brat and that her behavior is normal but she angers me each time she breathes. To make her likeable (to me), there's two solutions.

The easiest one consists in changing her personality altogether. With this solution, you could mold her as you wants. You could make her a reasonable person, make her thoughtful and logical. There're too many possibilities. But to make this behavior logical you would have to change her backstory. All future events would be influenced. If she was as logical as Sona for example, her decisions wouldn't be the same. Many times Sona would critic her actions and/or advise her on how to do it logically. If she were somewhat logical she would have heeded her advice.

But if someone was to make this changes, it would not be Rias anymore, only another character with the same name and appearance. It's like making Issei (yeah I also hate him) not a disgusting open pervert (self-righteous, plot-armoured, illogical, but a calm and sedate person. It wouldn't be Issei anymore. She would be out-of-character. Another thing I hate. If you want to make a fanfic at least try to keep the main characters behavior the same. If there's a change, it should be explained by the insertion of another character in the story or a trigger. (Eg: SI interacting with the character.)

All in all, you can understand that I don't particularly appreciate this solution.

The solution I would advise is keeping her backstory but making her a devious schemer. When I first watched the series, I thought that she was a devious mastermind that was just playing around everyone. I was disappointed.

It's simple. We just have to say that her normal personality is just a bluff, a deception and that she hadn't revealed her true personality yet.

There's one questions we should ask here: how to explain her behavior with her being a deceptive mastermind?

Answer: I always got all I wanted whenever I wanted since my childhood. I only had to throw a tantrum and everyone was quartering to my needs. I concluded at this time that being deceptive granted all my desires without me needing to do anything else. Little by little, I built my spoiled brat persona as a front to always obtain what I want. When I started my education, the study of Intelligent Being -Human- Relations helped me in reinforcing my persona. This thing that began as a childish way to please myself became my only way of life as I loved this feeling of control on others.

Even, the satan Lucifer (in fact he was the easiest to manipulate) would do anything for me as he was a big siscon. I knew that and I used it at its fullest. Some years ago, I was eager to find adequate peerage members (slaves). All I had to do was being adorable and asking sulkily to my dear big brother to help his lovely sister in my quest. A week later like an adorable puppy coming to its master, he came to me with a little fallen angel. She lost her mother, and came to be in Sirzechs care. All I had to do was being a good friend to her and I would have my first slave.

It was too perfect. I wondered if my big brother didn't orchestrate all this little show just to get this little slave. Soon, I didn't care anymore as even if he was a devious schemer himself he was wrapped around my pinky and would always do what I want.

I played my role of an impulsive child perfectly and made her my queen as soon as possible. Little after I discovered that I was betrothed to a little chicken named Riser. It wasn't a grave matter.

At this time, I found out one of the disadvantage of my little persona. I wanted to get stronger, stronger than my brother but I couldn't train as it would have been out of character.

Anyway, I was a strong girl and started to plan. If I wanted a legitimate reason to train, I have to put myself in a danger with the only solution being me and my slaves becoming stronger. This little marriage would be perfect.

I started to alienate the little teenager and he acted exactly as predicted. Ah, how it's easy to direct angsty teenager. He built himself a harem of slaves and I even contemplated doing the same. But not only would it be out of character, I also wanted only the best I could get.

I let my strongest slave find me my peerage member and he didn't disappoint me. I get a little cat (I even named it Koneko just for amusing myself), a dhampir and a chilvarous prick. It's at this moment that I noticed that my big brother interest in me was necessarily pure. All the male in my peerage was at best not interested in the opposite sex.

This was a deliberate action. Interesting. My little slave was trying keeping me to himself. I even found his attempt cool. At his place, I would have just sequestrated the object of my desires as I would be the strongest. Oof. Fortunately, he is not so rash. I might have pushed this a little too far.

My plan at this time was to find a little slave with a very big potential to counter him in this eventual case.

Some years later, I moved to Japan to claim a territory as it was becoming boring in the underworld. Unfortunately, I was accompanied by my 'best friend'. She was a big obstacle as she was smart and noisy. I had to push my act to another level to not be discovered by her. Fortunately, she had a certain view of me already and ignored my few lapses.

My marriage with Riser was approaching. I couldn't make him my little slave anymore but no need. I won't marry him. This ultimatum would give me the needed incencitve to begin training. Damned lazy persona. Anyway, even if I lose my dear brother would do anything to stop this little masquerade.

Let's see.

I win (unlikely) and then I decide to train to avoid future situations like this.

I lose. He cannot bear to let me to another man. He would try to negociate but me alienating him would make this negociations fail. What could he offer anyway? More girl? Political position? His wife? I laughed at the last one. He would then smite the poor boy and his family would react. They are not strog enough but it would be the spark that wold begin the 3 factions war anew. I could then gave up my lazy persona and become as strong as I could under his protection anyway.

I'm not losing anything.

Some time later my dear pet cat found a boy with a sacred gear that smeel like a dragon. He is an interesting boy. A disgusting pervert with no brain cell and amazing potential. He is just the perfect slave. How to play this one.

The fallen angels are killing all potential sacred gears users. I would let him die and reincarnate him as a slave. Yes, the simplest plans are the best.

Some times later, all happened according to my plan. It was a surprise that I had to use all my pawns to enslave him. His potential must be bigger than I thought. All I had to do was dropping my clothes for him to become a pathetic dog. Hmm, I choose well.

This guy is really something. Only some days and he brings me a new slave. With healing power and a meek personality. There should be someone high there that like me.

Now, as I planned we lost to Riser and I'm forced to marry him. I was a little surprised to seemy dg create new technicques but he is just wasting all his potential on his perversity.

Another big surprise. This little shit beats Riser. There won't be war. I have to change my plans.

Let's see. Hmm. I know.

This guy is too lucky. There should be something different here. I'll became his girlfriend after a long build up and let him recruit for me my slaves. He would be the strongest and I would be besides him directing him in the shadow.