Before attending Hope's Peak Academy, Mahiru went to middle school with her friend Sato, who joins the Reserve Course when Mahiru goes to Hope's Peak Academy. She formerly attended Light Forest Institute before going to Hope's Peak.
While in Hope's Peak Academy, Mahiru befriended Hiyoko Saionji, Mikan Tsumiki, and Ibuki Mioda, and introduced them to Sato. In addition, she had also joined a photography club. During the girls' time together, they discovered a corpse in the music room. The body was later revealed belong to Fuyuhiko's sister, who was murdered by Sato because the girl had been bullying Mahiru. Sato kept this fact a secret, blamed the murder on a pervert, and convinced the group not to tell the police they found the body. After the other girls left, Mahiru took pictures of the body. She found evidence in the next room that suggested Sato was lying to them about how she discovered the body. Mahiru took a picture of that as well before cleaning it up to protect her friend. The next day Mahiru showed her photos of the body to the other girls, saying they would be useful if the police questioned them, but Sato quickly snatched and destroyed the photos. Mahiru secretly held onto the photo of the evidence implicating Sato and confronted her with it the following day. She promised Sato that the evidence was cleaned up, then asked her what really happened. Sato admitted to a horrified Mahiru that she killed Natsumi, then ran away to dispose of the photo. The next day, Sato was killed by Fuyuhiko. Mahiru was seen crying on the floor after discovering her body. This part of her life was revealed in Monokuma's second motive, "Twilight Syndrome Murder Case".
Mahiru's Free Time Events reveal that she wanted to follow in her mother's footsteps as a photographer and gain her title of "Ultimate Photographer". She also had an unemployed father who didn't help Mahiru do the house chores when her mother was gone, and Mahiru says she likes to take care of him. Mahiru says that she feels inferior to the other students when comparing her talent to theirs. She also promised Hajime to give him her old camera and teach him how to take photos when they get off from the island.
