Mamoru is one of the twelve younger sisters in the Sister Princess series, born on October 18 and around thirteen years old. She is the athletic sister, characterized by her high energy, physical vigor, and natural talent for sports. Her appearance includes short brown hair and brown eyes, and she is almost always dressed in sporty clothing such as shorts and a bandana, often with solar goggles or other eyewear pushed up on her head—a trait that sometimes causes her to be mistaken for another sister, Rinrin. Mamoru is markedly tomboyish in both mannerisms and speech; she uses phrases and a form of address typically associated with boys, referring to her older brother as “Anii” or “Big Bro” rather than the more common affectionate terms used by her sisters. She embraces her tomboy identity and is not troubled by it, especially because her brother supports her unconditionally.

Her primary motivation is to spend as much time as possible with her brother, Minakami Wataru. She constantly seeks opportunities to engage in physical activities with him, whether it be running together in sports events, going on bicycle rides, teaching him how to swim, or simply camping in the outdoors. One of her signature skills is roller skating, which she often uses to move around quickly. Despite her comfort with sports attire, she feels embarrassed when she has to wear a dress, which highlights her lack of a traditionally feminine side. In the second anime series, RePure, Mamoru demonstrates her deep affection for her brother by planning a solo camping trip for the two of them, showing that she highly values one-on-one time and the chance to strengthen their bond.

Within the story, Mamoru fulfills the role of the energetic, sporty sister who brings a sense of activity and outdoor adventure. Her relationships with her other sisters are friendly and cooperative; though they all compete for their brother’s attention, they never display jealousy toward one another. Mamoru’s development is subtle but present—she becomes more open about her desire for exclusive moments with her brother and learns to balance her tomboyish nature with moments of vulnerability. Her notable abilities include remarkable athletic prowess in various sports, excellent roller-skating technique, and a confident, can-do attitude that encourages her brother to join in her activities. She is a loyal, affectionate sister whose love is expressed through shared action rather than words.