Evaness, Eva for short, is a young aberration that takes the form of a 10-12 year old  mermaid child. She, like all aberrations, is an undead soul that was revived by Miss Fortune and is bound to her to do her bidding. Evaness died in the water reservoir that appears in the pilot. Because of this, she manifests into the mermaid form she appears in. She has water and plant manipulation abilities. She's being guided by Theia (a transformed Luna) and has a little sister dynamic with her 

Evaness sounds like a whiny little kid who asks a lot of personal questions but never means harm. General "iPad kid" vibes.

For voice reference, look up The Collector in The Owl House, voiced by Fryda Wolff.