There's a ton of character archetypes that I like, generally it depends how they fit into their own personal stories which makes me like them. For example: Shinji Ikari from Evangelion is real shy and plain which usually wouldn't generate much interest, but he's surrounded by such an incredible cast of friends and dire circumstances that it leads to such interesting character interactions that you wouldn't see a shy or plain character usually have to deal with. I'm very much a huge fan of the "Brat" type, especially the actually somewhat antisocial ones rather than the teasing ones that you see in doujins. I'd say that Luka and Xiao are both in that "Brat" type, but they interact with the archetype differently, it leads them to coincide in some aspects and also butt heads in others. I'm also just a big fan of characters with obvious issues, it seems to generate different reactions in the audience and I think that's really neat. Some people go "I can fix him" others go "Wow, what a shit."