>>533694359
It's already been proven that Evil March is March's true personality and that her bubbly cute one is because her real memories are suppressed and sealed by very powerful means. Her breaking into Amphoreus thanks to the Garden's intervention gave her back her true memories undoing the seal.
Evernight is not March being possessed, it's March not being able to keep her true personality under control and taking control of her body.

She'll obviously have a way to revert back to her normal self, but according to the leakers that's who Evernight is. It's not possession, it's double personality.