Quella is the god of dreams. He is mentioned in the item descriptions for the Spirit Tree shields and the White Ring. Quella is either the subject of or the teller of an allegory that involves a blue spirit tree. The allegory (perhaps abridged) reads as follows:
Once there was a boy who was easily
frightened, until the talking tree of dreamworld
transformed itself into a shield to protect him.
Allegories can be interpreted to reveal a hidden meaning, typically a moral or political one and it would seem that this allegory has a moral meaning. The purpose of the talking tree of dreamworld becomes to protect the frightened and weak. Interestingly, protecting the frightened and weak is the cause of the Blue Sentinels. The White Ring also tells us that when wearing it, one is granted the protection of Quella, creating the illusion that the player is a white phantom and thereby confusing invaders. This also relates to the goals of the Blue Sentinels, as one of their aims is to protect the frightened from invaders.
The Spirit Tree Shields are capable of parrying magic spells, along with the Golden Wing shield. These shields are all related to the Blue Sentinels. So it may be that Quella and the Blue Sentinels are quite deeply connected.
Quella's symbol on the Name-engraved Ring is a cherub.