Mayu is a young homeless girl of around thirteen years old(In the manga she is introduced at age twelve and turns thirteen; but in the anime she is introduced at age 13 and turns 14.)
The viewer first meets Mayu when she arrives at the inn being inhabited by Kohta, Yuka and Nyu to return an umbrella she found at the beach. Mayu lives on the beach with stray puppy whom she refers to as Wanta; susbsiting mostly on bread crusts from a nearby bakery. When the bakery closes down though, she reluctantly accepts Kohta and Yuka's offer to stay at the inn.
It is later discovered that Mayu had run away from home after enduring repeated molestation by her stepfather. Her own mother would not protect her, because she was jealous of her.Khota and Yuka later contact her mother who seems eager to let her daughter stay with them; they are officially given custody over Mayu and she begins to go to school in their area.
Mayu is also responsible for saving Bando's life when she discovered him lying on beach dismembered and blinded after his first confrontation with Lucy. In the manga, it appears that Mayu either loved Bando or thought of him as a father