Umbra
Umbra is a benevolent shadow elemental that Ruethwyn can mentally summon using atavism. Umbra is not her full name.
Umbra is a fourth rank elemental. When joined with Ruethwyn via atavism, they are capable of equaling a fifth rank elemental.
Umbra doesn't like talking, not even to Ruethwyn. She likes men.
SPOILER HERE:
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Essryl knows something about Umbra, a secret which she keeps to herself. Essryl is terrified and exhilarated by the elemental.