Same word as quirk:

aberration, caprice, characteristic, conceit, crotchet, eccentricity, equivocation, fancy, fetish, foible, habit, humor, idiosyncrasy, id�e fixe, irregularity, kink, knack, mannerism, peculiarity, quibble, singularity, subterfuge, trait, turn, twist, vagary, whim, whimsy