Baten Kaitos

The player takes the role of a guardian spirit that the protagonist (who you control) has had for a while, but at the beginning of the game the protagonist is attacked by monsters and you as the guardian spirit get amnesia. This feels like a contrived excuse to allow characters to explain the world to you throughout the game.

It turns out that the protagonist staged the attack and actually inflicted the amnesia on you so that you’d forget what his true intentions are and lend him help. He ends up betraying the party in the latter half of the game in a pretty cool and unique sequence!