Mami, whose full name is Mamimi Yamagami, is a character who appears exclusively in the anime adaptation of Sister Princess, having been created for that medium and not present in the original source material. She is introduced as a fellow passenger traveling to Promised Island alongside the protagonist Wataru Minakami, and from the outset, her behavior is marked by an air of mystery. It is gradually revealed that she has been sent to the island on a mission by her older brother, Akio, who is a friend of Wataru. Akio, dissatisfied that Wataru was sent to the island rather than being accepted into a prestigious high school, instructs Mami to keep an eye on him and attempt to break him apart from the twelve sisters he has come to live with. To facilitate this, circumstances force her into posing as Wataru’s thirteenth sister, a role she is not actually related to, and she adopts the nickname Mami while using the pet name An-chan to address him.

Despite her initial role as a pseudo-antagonist carrying out a mission to separate Wataru from his sisters, Mami’s behavior is often played for comic relief rather than portrayed as genuinely antagonistic. As she spends more time living alongside Wataru and the twelve sisters, she begins to witness firsthand the deep affection Wataru holds for each of them and the genuine care they all share. This experience causes her perspective to shift significantly. She becomes less focused on her original task and grows into a supportive figure, offering honest opinions and advice to Wataru when he struggles with indecision or personal problems. Her motivation transforms from carrying out her brother’s orders to protecting the bond Wataru has with his sisters and even helping him find his way back to the island when he returns to Tokyo.

Key relationships in Mami’s story involve Wataru, whom she starts by observing but ends up genuinely caring for, and her brother Akio, whose instructions she ultimately reinterprets based on her own judgment. She also forms bonds with the twelve sisters, learning from them why they love their brother so deeply. Her character undergoes notable development from a spy acting on someone else’s agenda into a person who acts on her own conscience, becoming a trusted confidante. Mami does not possess any supernatural or extraordinary abilities; her notable traits are her resourcefulness, her ability to blend in and observe, and her evolving capacity for empathy and honest communication. In the second anime series, Sister Princess: Re Pure, she does not appear as a prominent character due to that series being more closely structured on the original magazine stories, but she does make a brief cameo working at a bakery.