Avan
I am not sure if I want her as a nemesis or a lover. Honestly I would rather see him somehow with Cleo or better yet a girl not of that planet. Arthur sort her out instead. Option 2 would also be interesting as it would keep more to the Merlin of old. Quite simply have him with no romance. Romance without purpose is never worth it. In fact due to a youth where he might experience trauma with women such as Gwen it would not be odd that he would choose a solitary lifestyle. Merlin provided wisdom and guidance and was ultimately a teacher and a guide. It really comes down to what you want for your character and how you invision your story. Is it more fitting for him to have a romance that remains unfulfilled the rest of his life? Does he get his happily ever after? If he has romance does he keep that part of himself separate from everyone else? Ultimately our main character will become someone at a higher stage. I could see him having some feeling build up but after Princess Gwen be far more hesitant towards developing feeling for others. While she didn't mean to harm him and time has past it is still tinged heavily in the memories of his father's death. Not to mention that he worked hard making a gift with his own two hands to impress her and years of friendship. Not to mention he keeps having betrayal after betrayal of the fairer ***. I could even see Morgana apprenticing under his previous mentor. Regardless of how you choose to portray it I look forward to what you develop. All I ask is if you choose romance please avoid harem. Choose a love interest that compliments him and his lifestyle and the character himself. Right now I think it is too early as you are still developing him. If he should have a child make them relevant to the plot. If he does have a love interest and she should die I imagine it would be very hard. Our MC belongs to a world full of immortal beings and losing your first love does not mean you can't find a second. You just love differently than the first and sometimes you cherish them even more for it. All I can say is thank you for the story and whatever you do please no harem!