maniac

madman, madwoman

abnormal, deviant

manic-depressive, monomaniac, mythomaniac, neurotic, obsessive, obsessive-compulsive, paranoid, psychoneurotic, psychotic, schizo, schizoid, schizophrene, schizophrenic, sociopath

character, codger, crack, crackpot, crank, eccentric, fantastic (also fantastical), flake, kook, oddball, oddity, original, quiz, screwball, tyke (also tike) [chiefly British], weirdo, zany

case, patient

groupie

admirer, amateur, collector, connoisseur, dilettante

authority, expert

adherent, convert, cultist, disciple, follower, hanger-on, votary

advocate, apostle, backer, champion, evangelist, exponent, friend, patron, promoter, proponent, supporter

partisan (also partizan), zealot

booster, rooter, well-wisher

faddist