Mamiya is a female warrior and the practical leader of her village in the post-apocalyptic world of Fist of the North Star. She was born to parents who built the village, and she grew up helping to protect it. On her twentieth birthday, the cruel Yuda murdered her parents and kidnapped her, branding his mark, the letters UD, onto her back. She managed to escape and, in order to survive and defend her home, consciously rejected her femininity and hardened herself into a capable fighter. She became the de facto leader of her village, making ruthless decisions when necessary but always acting out of a deep compassion for her people.

Mamiya fights using bladed yo-yos, emeici spikes, and a bowgun. She has no formal martial arts training on the level of the series' main fighters, but she is skilled and resourceful in combat. When the Fang Clan threatens her village, she hires Kenshiro and Rei as mercenaries. During this time, her younger brother Ko is killed by the clan, leaving her emotionally scarred but resolute. Mamiya bears a striking physical resemblance to Kenshiro's lost fiancée Yuria, which startles him when they first meet. She initially develops feelings for Kenshiro, but he does not return them.

Rei, however, falls deeply in love with Mamiya. He credits her with restoring his humanity after he had become consumed by a violent quest for his sister. In his final days, Rei dedicates himself to hunting down Yuda to avenge Mamiya's honor. After Rei's death, Mamiya honors his wish by giving up her life as a warrior and embracing her womanhood. She settles near Rei's grave, watching over it and living a peaceful life. She later reappears during the Gento arc and again in the final Bolge arc, where she helps Bat rescue Kenshiro and Lin, demonstrating that she has not lost her courage or her willingness to aid her friends.

Throughout the story, Mamiya develops from a defensive, emotionally guarded leader into a figure capable of love, grief, and renewal. She serves as a role model for the young girl Rin and remains a steadfast ally to Kenshiro and his companions. Her key relationships include her deep bond with Rei, her respect for Kenshiro, and her protective care for her village. Notable for her resemblance to Yuria, her weapon skills, and her arc from hardened warrior to a woman who finds peace, Mamiya is a significant supporting presence in the series.