hey! he sounds pretty cool ^^ I love that he makes his own clothes and stuff...
I'm curious about his family: did they treat him like royalty because he's so unusual and rare? Is his family normal demons and what are they like? Most demons tend to be more noble or rich anyways, but I'm curious if his family is middle class or rich and how they treated him. Does he have siblings?
Does he have a lot of friends? Does he treat them like equals,or subjects? If subjects, is he a fair and just ruler and do his friends actually let him think he's a Prince, or do they tease him about it? I kind of assume if he likes games that require real interaction a lot, he probably has friends who are into dressing up and RPGs too!
What about things like pets, can an admiring canine companion provide him with enough of an ego boost if there's no people around? Is he good at commanding animals, or just "people" that are sentient beings?
I think you may wish to not use the crown for attacks unless it is actually a part of his body, because I think Reapers are the only characters that can use items as part of their attacks? Maybe it is part of the horn that grows into the shape of a crown, rather than an object not attached to him?
He's arrogant- I'm curious what he does if someone insults him? Would he get mad, get even, or ignore them? Or all three in some way? XD Does he play favorites with his friends and make his closer friends more like advisors? Does he like playing politics and interpersonal drama, or would he much rather to do his hobbies and play games?
How does water relate to his nature aside from gills and fins? Can he swim well, does he ENJOY water or does he NEED to be in it to keep moist?