I like all of my characters with no exceptions but if I had to choose one, it would be Debbie, my first NPC from Trainers. The reason? I don't know, really, she's that character that is there for no reason other than adventure itself and is literally up to wrestle with a wild Pokémon just because she thinks it's fun. She's fun and easy to play, I can write all kind of nonsense with her and it still won't feel off somehow.
If I had to choose a main character it would be Carmen, though. I was kinda experimenting with her when I wrote her and that's probably the reason I like her that much.