The two who split
Gods
One of them made the other. They disagreed about a single question — whether a creator should stand at a distance or live among the created — and rather than destroy each other they divided everything.
Supreme Architect · Earth 1
Zalian
The original creator. She holds to a law she wrote for herself: she does not read a conscious mind. She hears only what is said aloud, and what is dreamed — because in REM the boundary between thinking and speaking dissolves.
It is a rule made out of respect for free will, and it is the crack the whole theft runs through. She has always known there is foreign DNA in one of her humans. She let him exist anyway.
The Firstborn · Earth 2
Balian
Zalian's own creation, grown past what she intended. He wanted presence: to stand inside his world rather than above it. He took followers and left, and on the far side of the Barrier he walks among the Virden in person.
He understands Zalian's architecture better than anyone alive, including the exact shape of her blind spot. Whether he engineered the flaw or merely noticed it is the question the story keeps asking.