Mamoru is a child from MegaMan Battle Network 3 who, like Hub Hikari, has the heart condition "HBD". He is hospitalized since he has three years old. Remarkably, he survives what most people wouldn't. When Lan met him on the beach for the second time, he has an HBD attack, which Lan witnessed. Lan reenters the Seaside Hospital and finds his doctor, taking him and a nurse to help him. This attack landed him in the emergency room, but he didn't want a surgery. His nurse tells Lan he underwent surgery three times, but those were all failures, apparently. Mamoru has no hope that a fourth surgery would solve his problem, even though it is a new kind of surgery for HBD, but Lan encourages him and gave him a IceBall M Battle Chip.
Three days later, Mamoru's condition got worst and he was sent to the emergency room for his fourth surgery, but it was interrupted three hours after the begin of the surgery by Anetta, who was disguised as a nurse and used her NetNavi to make the growth function of the Tree of Life, a large tree in the middle of the hospital, malfunction and create vines through the hospital to destroy the place and find a TetraCode for Wily. After MegaMan defeats PlantMan, the Tree of Life returns to normal and Mamoru's surgery is successful. He returns to his house after this, but it's unknown if he was cured or just better. No information is revealed regarding Mamoru's future. Though a scene in the end credits show Lan giving Mamoru a PET, suggesting that he was well enough to NetBattle. He appears again later in the game to help Lan.It is believed that Serenade is Mamoru's NetNavi, since Mamoru's father created the Undernet and he has Serenade's emblem on his wheelchair.