Idiosyncratic

/ˌɪdioʊsɪŋˈkrætɪk/ adjective medium

Peculiar to a specific individual, reflecting a personal quirk rather than any general pattern.

His idiosyncratic habit of alphabetizing his spice rack baffled his roommates.

Memory Trick

'Idio-' means 'one's own' (as in 'idiom') — an idiosyncratic trait belongs to one person alone, a quirk nobody else shares.