Sumi Ichijouji is the head doctor of the flying battleship Amanohara, holding a senior medical position responsible for overseeing the health of all crew and Lime Unit members. Her expertise is vital to the ship's operations.

She persistently pursues a romantic relationship with Shintaro Umakai, the civilian commander and teacher of the Lime Unit. Her active attempts to seduce him frequently create tension with his teenage students, who are also competing for his attention. It is suggested Sumi was the first female crewmember aboard the Amanohara to become sexually involved with Shintaro.