April's default human form is as a tall, lithe woman of indeterminate age. Her hair is glossy black with white streaks, and she normally wears it long and loose; when she is upset, it moves independently until it becomes a part of her alien form. Except for her hair, she bears a recognizable family resemblance to both of her human parents.

In her alien form, Mayhem appears to be a black-and-white symbiote host whose white spots flow across her surface like oil in a puddle of water. Her hair resolves into writhing tentacles, and her fingers elongate into razor-sharp talons over a foot long.

Interacting with her in either human or alien form easily reveals that she is both deeply psychologically disturbed and attempting to hide something. Sometimes she seems extremely volatile and impulsive, while other times she seems much more thoughtful. She is protective of innocents and especially children, but tends to have cotentious relationships with other supers-- particularly Spider-Man and his clones, any member of the Osborn family, and anyone bonded to a klyntar symbiote.