Kouichi Tsuruuchi is a senior Garde pilot and a member of the Samari squad aboard the seed ship Sidonia. His demeanor outside of combat is notably easy-going, and he often makes rude jokes at the expense of those around him, including his commanding officer Samari Ittan. This abrasive humor masks a professional focus that emerges sharply during battle, where his attitude shifts into that of a skilled and determined pilot. He is recognized as one of the few people who initially believed in and supported Nagate Tanikaze’s abilities as a pilot, working alongside him during critical operations against the Gauna.

Tsuruuchi’s primary motivation is the survival and defense of Sidonia and its inhabitants. He approaches combat with a calm intensity and has demonstrated quick tactical thinking when coordinating with fellow pilots or adjusting to unpredictable Gauna behavior. Over the course of the Battle for Planet Nine arc, his role expands beyond routine squadron duties. He becomes entangled in the political friction surrounding the development of a new, experimental Garde type, voicing concern over the weapon’s potential risks. This places him at odds with some in Sidonia’s leadership and highlights a pragmatic, principled side to his character that prioritizes ethical caution alongside military effectiveness.

Key relationships include his direct connection to Samari Ittan, whose authority he teases yet respects in combat, and his supportive rapport with Nagate Tanikaze. While Tsuruuchi remains a supporting figure rather than a protagonist, his loyalty and frankness contribute to the cohesion of his squad. His development illustrates a shift from a seemingly crude jokester to a reliable pilot who is willing to question dangerous decisions, emerging as an independent voice of reason amid the broader fleet politics.