I would only use it on a boy. It's not a unisex name in that it isn't a surname or a word or place name and has been long-established as a male name.
Tristan Alexander
Tristan Blake
Tristan Charles
Tristan Elliott
Tristan Isaac
Tristan James
Tristan Joseph
Tristan Lawrence
Tristan Michael
Tristan Oliver
Tristan Patrick
Tristan Robert
Tristan Scott
Tristan WIlliam