Guardian Gaist is a character in Breath of Fire III.
After the party pray at Angel Tower, they are told that they must release the woman trapped inside the nearby energy pyramid before they can meet God. To do this, they must meet Guardian Gaist.
The party travel to Gaist's new home. There, he asks to speak to Ryu alone, before attacking him. After the battle, with his dying breath, he tells Ryu that his death was the only way to open the energy pyramid.
With Gaist defeated, the party head back to Angel Tower. As they approach the chamber, the woman is released. She asks Garr to step closer, before unleashing five hundred years of fury on him.
Before the game began, a great war was fought, between the Guardians (who look astonishingly like the monsters Myria almost unleashed in Breath of Fire II) and the Dragon Tribe (otherwise known as the Brood). Deis, as she introduces herself, stood against the Guardians and Myria, and was trapped inside that chamber by Gaist because of it. She has been stuck there for the past five hundred years, placing this game at least six hundred years after Breath of Fire II - probably longer.