The Kissi are a colony of demonic creatures from before the dawn of time. Aelita says, "Kissi were born in chaos. It's what they're made of. It's what they consume. Humans create a particularly appetizing sort of chaos to the Kissi. Eons ago, there was a war between us angels and the Kissi that raged from the earth all the way to the gates of Heaven. Neither side won." This war led to the formation of the Golden Vigil.
Stark calls them "a race of deformed, half-finished angels that lived in the chaos on the edge of Creation. One of God's first great fuckups while creating the universe." They smell like vinegar.