Ichiro Seii serves as a senior guardian pilot aboard the Sidonia, initially holding the roles of executive officer and battle commander aide to Captain Kobayashi. He delegates officers during military operations in the Combat Info Center, securing Kobayashi's approval for major tactical decisions. Later transitioning to a dedicated gardes pilot, he recommends Yuhata Midorikawa as his successor. As an instructor at the Sidonia Defense Force academy, Seii teaches tactics and Gauna-related courses to train future guardians. Recognized among the top pilots, he survived the 5th Gauna Defense War. During his first encounter with Gauna 487 on a mining mission, fear paralyzed him, though his subsequent assistance in executing a diagonal acceleration maneuver prevented greater casualties. Seii leads the Seii squad, designated unit 026. His combat record includes critical engagements like the assault on the Large Mass Union Ship, where his squad suffered heavy losses—38 of 48 pilots perished—yet eliminated over 200 Gauna and saved Sidonia. His tactical style favors direct engagement, frequently clashing with Kobayashi over strategy. He mentors Nagate Tanikaze, refining Nagate's distinctive fighting technique with the Tsugumori Garde. Physically tall with white hair and blue eyes, Seii initially wears the standard crew uniform before adopting a skin suit as a full-time pilot. In the manga, he is depicted wearing a vintage helmet covering most of his face. Seii originates rumors among pilots, including one about Samari Ittan photosynthesizing with high-performing male pilots to boost morale. This interaction suggests a deeper connection, evolving into a relationship with Samari and resulting in several children. His surname "Seii" translates to "power," echoing his authoritative role, while his given name "Ichirou" combines "one" and "son."