The type of fan who's intrested in only ONE character (usually because they find them attractive) and hate the rest of the characters in the same work/franchise.
-Hey, who's your favorite character in this game? John? Thomas? Alice?
-Idgaf about neither of them, I'm only here for Nolan, he's like the only person I actually enjoy watching
-Ah, so you're oni-fan then
A word typicaly used in Japanesse animes by younger female animegirls to their older brother or just by anoying weeaboos who think they know the whole damn language.
"Oni Chan (big brother) can you make me a glass of milk?"
"Oni Chan (big brother) can I watch TV with you"