Mamoru Kusanagi is a central character whose origins are deeply intertwined with the Aragami, ancient monstrous entities that threaten Japan. As an infant, his parents were killed by the Aragami lord Orochi, who implanted seven mitama, or Aragami souls, into his body. Orochi assigned him the lifelong mission of protecting the Kushinada, the designated human sacrifice whose power can put the Aragami into a dormant state. Growing up, Kusanagi watched over Kaede Kunikida, the first Kushinada, and developed complicated feelings for her. He was deeply unhappy with his forced destiny and sought a way to free himself from the Aragami by killing the Kushinada, believing that her death would cause the pulse that locks the Aragami in sleep. After Kaede's apparent sacrifice, he attempted to kill her twin sister, Momiji Fujimiya, but found himself unable to do so because her face reminded him of Kaede and triggered an impulse not to harm her.

Kusanagi possesses a rough, blunt, and often crude personality, frequently teasing Momiji about her appearance or choice of underwear. Beneath this abrasive exterior lies a tortured individual constantly fighting the Aragami influence inside him. His primary motivation shifts from seeking personal freedom to genuinely protecting Momiji and helping her fight the Aragami. He becomes a key member of the Terrestrial Administration Center (TAC), an organization dedicated to battling the Aragami, and takes on the role of Momiji's protector. Over the course of the story, he develops a deep romantic bond with Momiji, who recognizes the pain he carries and eventually confesses her love to him. Kusanagi also shares an unusual connection with Momiji because both received mitama from Orochi, which becomes crucial in the later stages of the conflict.

Kusanagi's abilities are derived from the seven mitama implanted in his body. His mutation gives him green blood and the power to transform his body to some extent. He can sprout sharp blades from his arms, and he possesses extraordinary physical abilities, including impressive jumping and agility. However, without an eighth mitama, he initially lacks the strength to directly confront the powerful Aragami Murakumo. Late in the series, he receives an eighth mitama from Momiji when he rescues her from danger, which grants him enhanced power and allows him to fight Murakumo on equal terms. Throughout the story, Kusanagi evolves from a reluctant servant of the Aragami into a devoted protector and ally, choosing to fight for both Kushinadas and for his own freedom. His name carries symbolic meaning: "Mamoru" means protector, and "Kusanagi" is derived from the legendary Grass-Cutting Sword of Japanese mythology.