hardcore enthusiast; fanatical; geeky; specialized
Used as a noun for an intense enthusiast and especially as a na-adjective for people, tastes, questions, information, etc. that are very specialized or appeal to enthusiasts. It often has a casual, slightly amused tone rather than the severe English sense of 'maniac'.
His taste in music is quite specialized.
The clerk answered even the geeky questions carefully.
He is quite a hardcore enthusiast and even remembers the model numbers of old lenses.
He has a really geeky hobby.
She's a real hardcore enthusiast when it comes to trains.
He's really obsessive about anime lore.