Mamoru Kusanagi, most often called simply Kusanagi, is a central character in the anime Blue Seed. His origins reach back to his infancy, when the ancient entity Yamata no Orochi killed his parents, who were Shinto shrine priests, and forcibly implanted seven mitama, or Aragami souls, into his body. Orochi then charged the child with a single task: to watch over the Kushinada, the sacrificial maiden whose blood could send the Aragami back to sleep, until the monsters were prepared to act. Kusanagi grew up secretly observing Kaede Kunikida, the Kushinada who had been raised by the TAC director, and through that long vigil developed a complicated emotional attachment he could not fully control.

When the story begins, Kusanagi approaches Momiji Fujimiya, the twin sister of Kaede, with the intention of killing her. After Kaede’s apparent death, he believed that ending the Kushinada line would free him from servitude to the Aragami and grant him a measure of independence. That plan is thwarted, and over the course of the early episodes he abandons his original murderous aim. From that point forward, Kusanagi dedicates himself entirely to protecting Momiji from harm, a shift that reshapes his entire existence.

His personality is shaped by a deep-seated resentment of the destiny forced upon him. He is aloof and sharp-tongued, often teasing Momiji about trivial things such as her choice of underwear in order to fluster her, yet beneath that sarcasm is genuine devotion. Kusanagi is fiercely determined and refuses to accept defeat, even when outmatched by foes like Murakumo, an enforcer with eight mitamas who slays renegade Aragami. Though he can appear cold, his protective instincts are unwavering, and he gradually reveals a caring and emotionally vulnerable side.

Kusanagi’s primary motivation is liberation: first through a desperate plan to kill the Kushinada and later by destroying the Aragami masters that bound him. That motivation evolves into a personal mission to keep Momiji alive at all costs, a goal that becomes inseparable from his growing love for her. His internal conflict centers on the tension between his desire to be free and the ties of duty and affection that bind him to the Kushinada.

In the larger narrative, Kusanagi acts as Momiji’s self-appointed guardian and a powerful ally to the Terrestrial Administration Center, or TAC, even though he is not an official member. He repeatedly confronts Aragami threats and faces Murakumo in an ongoing struggle. The relationship between Kusanagi and Momiji forms the emotional core of the series: their bond deepens through shared danger, and Momiji confesses her love to him during a critical moment, to which he responds by asking her to wait for him. His earlier closeness with Kaede also haunts him, coloring his initial interactions with her sister.

Physically, Kusanagi is no longer fully human. The seven mitama fused with his body have given him green blood and the ability to mutate in combat. He can sprout blades directly from his arms and possesses extraordinary physical abilities, including the strength to leap great distances and heights. Without an eighth mitama, however, his power remains incomplete. In the final stages of the conflict, Momiji transfers an additional mitama to him, granting him the full strength necessary to face Murakumo on equal footing. That gift symbolizes the culmination of his character arc: the acceptance that his fate is not only about shedding a burden but also about embracing his connection to the person he loves and the role he has chosen as a protector.