One of doctor Zorndykes creations. She became friends with Tetsu Hayami.
She pilots one of the Spiders, mecha amphibian battle ships, that attacked the Blue N6 for the first time. Tetsu and Mayumi manage to disable her Spider, and she tries to scape but its seriously wounded. Mayumi aims her shotgun towards her but Tetsu stops her and he grabs Mutio and leaves her inside the water, saving her life. She goes back to the Ghost Ship, the main battle ship of the Sea Army commanded by Verg, another hybrid created by Zorndyke. When the Ghost Ship attacks the underwater Blue main base and Tetsu´s grampus gets hit, she goes after Tetsu and saves him by taking him over one of the floating remaining´s of the Blue base. When Tetsu wakes up she brings him some fishes to feed him and stays along his side. After a wile the hybrid companions of Mutio discover Tetsu and try to kill him, despite Mutio´s effort to prevent it, but a kindhearted Musca save both of them and take them near the Blue N6, so Tetsu can join his partners again. The Blue N6 attacks the Musca believing his an enemy, Mayumi takes Tetsu back inside the submarine again wile Mutio stay´s alongside the Musca till its final breath. After this Mutio goes back to the Ghost Ship again, but its severely punished by Verg, for helping Tetsu. She´s kept inside the Ghost Ship till its destroyed by the joint forces of all the remaining submarines. After this event she escapes and goes to the sea shore of the South Pole, were she meets Verg again. He had tried to kill Tetsu after he discovered that Zorndyke had died, but stopped after seeing Mayumi´s tears and realizing that humans are also able to feel emotions. Mutio consoles him and they part to the deeps of the sea.