Ichirō Seii is a senior guardian pilot and a key officer aboard the spaceship Sidonia, known for his significant experience and leadership within the Sidonia Defense Force. In the events of the series, he initially serves as the executive officer and battle commander aide to the ship's captain, Kobayashi. His primary responsibility in this role is to direct military operations from the Combat Information Center, delegating orders to officers and coordinating tactical movements, with major decisions requiring the captain's final approval. Despite being a capable tactician, Seii comes to believe that others may be better suited for command at the operations level. True to this self-assessment, he eventually steps down from his post, recommending the younger and highly skilled Yuhata Midorikawa as his own successor, and transitions to becoming a dedicated Garde combat pilot. He also serves as an instructor at the SDF academy, where he teaches courses on tactics and the nature of the Gauna to prepare future generations of guardians.

Seii is presented as a serious and pragmatic veteran whose personality has been shaped by the constant threat of humanity's alien enemy, the Gauna. He is one of the few pilots who has survived the Fifth Gauna Defense War, a testament to his durability and skill. However, this experience does not make him immune to fear; during his first encounter with the Gauna designated 487, he was struck numb by the horror of the situation. Yet, even in that fearful state, he contributed to evasive maneuvers that, while resulting in casualties, saved the Sidonia from complete annihilation. This incident highlights his capacity to function under extreme pressure. He is shown to be a direct individual who is not afraid to clash with Captain Kobayashi over tactical decisions, favoring strategies of direct engagement. He also exhibits a pragmatic, if unconventional, approach to morale, as he is the one who initially spreads a rumor that pilot Samari Ittan boosts the fighting spirit of high-performing male pilots through a process called photosynthesis. This act is implied to have had the desired effect of increasing pilot motivation.

Seii’s role in the story evolves from a frontline commander to a combat pilot and mentor. One of his most important functions is as a supporting figure to the protagonist, Nagate Tanikaze. Seii assists in bringing out Nagate's natural talents, helping to refine the young pilot's unique fighting style with the Garde unit known as the Tsugumori. His shift in position is not a demotion but a strategic choice that allows him to apply his skills directly in battle. He leads his own unit, designated Seii squad, and participates in some of the most dangerous missions. During a critical assault on a massive Gauna Union ship, his squad suffers devastating losses, with thirty-eight out of forty-eight pilots being killed. However, in that same engagement, they destroy over two hundred Gauna, playing a decisive role in saving the Sidonia. This event encapsulates his leadership style: demanding, high-risk, but capable of producing crucial victories at a terrible cost.

On a personal level, Seii’s key relationships include his professional respect for Captain Kobayashi, despite their disagreements, and his mentorship of both Yuhata Midorikawa and Nagate Tanikaze. The rumor he spreads about Samari Ittan eventually gives way to a more genuine connection, as it is implied that he enters a long-term relationship with her and together they have several children. Regarding his notable abilities, Seii is consistently ranked as one of the top Garde pilots aboard the Sidonia. His combat effectiveness is not just in his piloting skill but in his ability to analyze situations and adapt, whether as a commander or a frontline fighter. He is tall, with distinctive white hair and blue eyes, and wears a typical crew uniform before his reassignment, after which he adopts a form-fitting pilot suit. In the original source material, he is also known for wearing a vintage helmet that covers most of his face.