A being that watches over those designated Spiriters in the Baten Kaitos series. In reality though it is actually the player. In both Eternal Wings and the Lost Ocean and Origins Sagi/Kalas occasionally addresses the spirit by turning toward the screen and asking him/her to make certain decisions. These decisions affect the relationship and connection that the characters have with the spirit in battle this manifests by what magnus come up in battle. In the case of Sagi, though, he didn't have a true guardian spirit as it was actually an essence of Malpercio due to his status as part of the Maledeiter project.
In Eternal Wings and the Lost Ocean, the Guardian Spirit is represented as wild cherry blossom petals.
For example: if you make the right decisions as the guardian spirit then you will have a better chance of getting a needed card such as health if you're low or a powerful spirit attack.
Aside from the player characters, Baelheit was shown to have a guardian spirit late in the game called Daimon. Quaesor Verus also claimed to have a guardian spirit, although it was later revealed that he had been faking it all along, or at best, that said guardian spirit was in fact Wiseman.