With a 2nd soul you need a 2nd body-Esuiluhn

In The Grand Ceremony Part 7 Metayel wanted to give the demon, Esuiluhn, a body of her own instead of staying inside of Alexis. Unfortunately he couldn't, but he was able to connect Alexis and Esuiluhn's souls together. This allows them to communicate with each other and switch between who controls their body. (Esuiluhn mainly takes over when they're in danger.)

Extras:
1: Whenever Esuiluhn takes over, she changes the way Alexis looks to show that she's not of this world.
2: Alexis treats and talks to Esuiluhn as a sister since they basically inhabit the same body.

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PS Esuiluhn is pronounced Es-Swee-Loon.