General synonyms:
* Peculiarity
* Quirk
* Eccentricity
* Whimsy
* Habit
* Trait
* Characteristic
More specific synonyms:
* Foible: A minor weakness or eccentricity in someone's character.
* Idiosyncratic: Adjective meaning "peculiar to an individual."
* Singularity: A unique or distinctive feature.
* Distinctive feature: A characteristic that sets someone or something apart.
* Oddity: Something unusual or strange.
* Mannerism: A distinctive way of behaving or speaking.
The best synonym will depend on the context and the specific meaning you want to convey.