Sin Eaters

[Cilia]

Just as a random off-the-top-of-my-head thought, it's possible that Sin Eaters are a semi-natural part of the First's ecosystem, and only became malevolent and actively hostile with the rising of the Flood. After all, Kuribu, the winged lion, and Innocence all look rather normal unlike the twisted creatures featured at the Keynote, so they could have just been normal beings that could pop onto the Source at random like voidsent via natural voidgates.

Voidsent themselves are unnatural, though...