Mayu is a 14 year old runaway who lives with Kouta and company, being one of the first main characters to see who Lucy really is.
In the anime, Mayu is first introduced in episode 2 right before the ensuing fight between Bando and Lucy. She was living in quite literally a cardboard house with a runaway puppy she named Wanton. After the fight between Lucy and Bando where Bando loses an arm and his sight, Mayu emerges to see what the fuss was about and tends as best she can to Bando's injuries. She runs to call an ambulence but finds out he had crawled away when she returns. Finding an umbrella that same night that is marked Kaede House, she goes to return it, meeting Kouta, Yuka, and Nyu. They ask her to stay and eat as Kouta wishes to know about what happened between Lucy and Bando, but Mayu admits that she saw nothing.
Soon after, Mayu witnesses a fight between Lucy and Nana. Remembering Lucy from the Kaede House and seeing the two fight, she beckons the two to stop. She stops short when she notices how Nana's leg has been ripped off and Lucy uses her vectors to push Mayu into a nearby tree (probably not an act of mercy, but rather to get her out of the way). Mayu shakes the whole thing off as a hallucination and is picked up by Kouta, who claims the two are cousins. Kouta and Yuka implore that Mayu stays for a while as they may have questions about Nyu, but Mayu leaves the next day.
As Mayu leaves the house, her back story is revealed. Her father having died or divorced (not mentioned in the show), Mayu's mother remarries to another man who Mayu is skeptical about. Her doubts are correct as we see her sexually exploited by her stepfather numerous times and when brought up to her mother, she is beaten as her mother is jealous that her stepfather spends more time with her daughter. This continues until Mayu runs away, soon after meeting Wanton. As she tries to live by herself, the police arrive wondering what she was doing out by herself as she runs into Kouta and Yuka, proclaiming again that they are her family and take her home. Mayu eventually agrees to stay at the Kaede house and goes to school.
She later runs into Bando who she accidentally tells him that she knows of Lucy but cashes in a favor Bando had given her as he threatens her life. Mayu decides not to tell anyone and then meets Nana, who she remembers from her "hallucination". She is shocked to see Nana's artificial limbs falling off and passes out, waking up to see Nana burning money (as she thought it to be useless paper). She invites Nana to stay with her at the Kaede House, but Nana attacks Lucy on sight and is kicked out. Mayu chases after her to which Nana reveals her Diclonyius heritage and vectors, explaining how what happened at the graveyard wasn't a dream. The two go back as Nana realizes the difference between Lucy and Nyu and things are fine.
Towards the end of the anime, Mayu exclaims how she considers everyone to be her actual family, never at once telling anyone what actually happened to her. During the penultimate episode, she and Yuka talk, Yuka explaining why Kouta wants to help her and Mayu's feelings towards everyone, considering Nyu/Lucy and Nana as sisters while Kouta and Yuka are like her father and mother respectively. At the end of the last episode, she is more of a backround character as she is trying to teach Nana to cook, truly happy with her new life.