Y entered the Latin alphabet later and by a different route than most of its neighbours. Classical Latin's native alphabet, inherited from Etruscan, had no letter shaped like the Greek vowel upsilon; when Rome began transliterating large numbers of Greek loanwords and proper names during the late Republic, its scribes borrowed upsilon directly from Greek, rather than adapting an existing Etruscan-derived letter, and placed the new letter at the very end of the alphabet alongside Z rather than inserting it into upsilon's original Greek position, roughly where U and V already sat.
Because of this late, direct borrowing, Y in classical Latin usage was largely confined to Greek words, such as the many technical and philosophical terms Latin absorbed from Greek, and Roman grammarians noted its foreign character explicitly, sometimes calling it the Greek y for the difficulty native Latin speakers had pronouncing the rounded Greek vowel it represented. Its graphic shape, a V or U form topped by a shorter stem, has changed little since that Republican-era borrowing, and Y carries no Roman numeral value, since it entered Latin too late and too narrowly to be absorbed into the numeral system built around the alphabet's older core letters.