The series' main protagonist. Despite having a hard time adapting to Sidonia's way of life and getting in trouble constantly while being rejected by other humans due to not being aware of the laws and attributes of the new culture, he eventually becomes the spaceship's population's savior, main defender, and hero over time. He pilots an older but capable Guardian mecha model called "Tsugumori."

Unlike most Sidonians who were genetically engineered to have the capacity to receive most of their energy requirements through photosynthesis and only need to consume food for only once a day, he is a pure human who has to consume adequate amounts of foods to satisfy his energy and nutritional needs.

He is the clone of Sidonia's hero Saitou Hiroki who was the original Ace of Sidonia. When he was cloned, they made changes to the genetic information so that he became immortal from birth, unlike the other immortals who were later manipulated, and grants him an abnormal healing which might explain the need to consume large amounts of food daily for his extended metabolism and healing.

He received elite training since childhood in order to be able to deal with all kinds of environments, situations, and even weapons in their prototype phase.