She is a young 13 year-old girl (14 after episode 4), abandoned by her mother and a victim of sexual abuse by her stepfather. She has run away from home and is homeless at the start of the series, subsisting on bread crusts she is given at a local bakery under the pretence that they are for the small puppy that accompanies her. She found and named the puppy "Wanta." She ends up living with Kouta and Yuka as her legal guardians. Mayu befriends Nana and tries to take her under her wing.
Like Kouta, Mayu carries the scars of what she has been through. Besides the direct abuse of her stepfather, she suffered further when her mother treated her not as a victim, but as a sexual competitor and threat to her marriage. Especially in the anime, she sees Kouta and Yuka as her parents, for all their flaws, being worlds better than the house she fled. In the manga, she briefly becomes suspicious of Kouta when one of Nyu's antics creates an awkward situation, but later seems to recognize that Nyu's affections are as uncontrollable as Lucy's rages.