Koichi Tsuruchi is a senior Garde pilot in the defense forces of the seed ship Sidonia. He initially served as a member of the Samari squad, where he was known for his lighthearted and easygoing nature. This easygoing demeanor often manifested as crude or flirtatious jokes directed at his commanding officer, Ittan Samari, a running gag that typically ended with Tsuruchi being physically rebuffed but unswayed in his teasing. He later becomes the leader of his own squad, designated Tsuruuchi squad.

His personality presents a stark contrast between his casual, joking surface and the serious reality of combat against the Gauna. While he provides moments of levity and camaraderie among the pilots, he is fundamentally a dedicated and capable soldier whose underlying motivation is the protection of Sidonia and the survival of its people. He serves as a veteran presence alongside the main characters, offering experienced support during high-stakes missions.

Tsuruchi's defining moment occurs during the conflict involving the Chimera, a formidable hybrid entity. Recognizing the existential threat it poses, Tsuruchi makes the ultimate decision to sacrifice himself. His final act is a suicide charge against the Chimera, a moment where his characteristic humor is stripped away to reveal the profound sense of duty and courage that defines him. He explicitly justifies the sacrifice to his comrades with the conviction that failure means the end for everyone, cementing his role not just as comic relief, but as a deeply committed hero willing to lay down his life in the line of duty. As a pilot, he is skilled enough to lead a squad and participate in major campaigns, such as the expedition to the ninth planet of the Lem system.