Mayu is a young teenage girl who first appears in the series as a homeless runaway. She is approximately thirteen years old at the start, later turning fourteen. Her background is marked by severe trauma: after her mother remarried, Mayu was subjected to repeated sexual abuse by her stepfather. When she tried to confide in her mother, Mayu was rejected and blamed, her mother treating her as a rival rather than a victim. Ultimately, Mayu fled her home and has been living on the streets, surviving by collecting food scraps from a local market under the pretense that they are for her dog, a small puppy she found and named Wanta. Her motivation throughout the story is to find safety and belonging, having been denied both by her family.

Personality wise, Mayu is initially very shy, withdrawn, and distrustful of adults, a direct result of her abuse. She is quiet and avoids drawing attention to herself. Despite her hardships, she retains a gentle and caring nature, which is most evident in her devotion to Wanta and later in her friendship with Nana, another displaced girl. Mayu is also resilient, having learned to survive alone, but she craves affection and stability. Her primary motivation is to escape the pain of her past and to find a place where she is wanted, which she eventually begins to discover at the Maple House.

In the story, Mayu serves as a key supporting character. She is introduced when she encounters Kouta after Lucy injures a soldier, and she helps call an ambulance. She later becomes involved with the main group after walking in on the conflict between Lucy and Nana. Kouta and Yuka take her in, initially for a night, but when she runs away out of fear of being a burden, they seek her out and eventually become her legal guardians. Mayu's role is to represent the theme of human cruelty and the possibility of recovery through kindness. She also acts as a bridge between the human and Diclonius worlds, forming a close bond with Nana, who is herself a persecuted Diclonius, and teaching Nana how to survive on the streets.

Her key relationships include Kouta and Yuka, whom she comes to see as parental figures, flawed but far more loving than her biological family. She also becomes a friend and mentor to Nana, showing her compassion and guidance. With Nyu, Mayu develops a simple friendship, bathing together and enjoying moments of normalcy. Mayu has no supernatural abilities; her strengths are emotional rather than physical. Her growth is seen in her gradual ability to trust and accept care, moving from a frightened, homeless girl to a more secure member of the Maple House household. By the end of the series, she remains living with Kouta and Yuka, having found the home and family she desperately needed.