It is a ghostly humanoid with long arms, sharp claws, two big horns, enveloped in white mist.
Wraith's elemental weakness and resistance change from battle to battle. It starts by casting Doom, and if no one is doomed on its turn, it may cast Death. It is one of the only enemies from which Kimahri can learn the Doom Ronso Rage.
Summoning an aeon is wise as they are immune to Doom and Death. An aeon like Shiva or Ifrit is a good choice, since they can heal themselves. Delay Attacks can also be used, and the party can win the fight without the Wraith even getting a turn.
Wraith is a Scottish dialectal word for "ghost," and/or "spirit".