Yasuo Nanbu serves as an officer in the tactical division aboard the space battleship Yamato during its mission to Iscandar, holding the rank of Lieutenant Junior Grade (Second-grade Space Lieutenant). As chief gunner, he commands the deployment and performance of the ship's primary and secondary weapons, including cannons and missiles, directly supporting combat operations under tactical division leader Susumu Kodai.

Heir to Tokyo-based Nanbu Heavy Industry, a major Earth munitions manufacturer, Yasuo possesses family-derived technical expertise in weapons systems. Defying parental objections, he joined the Yamato Plan when selected as crew, prioritizing duty over family expectations and creating persistent strain with his father, Kozo Nanbu.

Following the Yamato's return, Yasuo transfers to the supply carrier Asuka (DAOE-01) in 2205. Promoted to First Lieutenant, he assumes the roles of Executive Officer (XO) and Chief Tactical Officer. Reporting directly to Captain Yuki Mori, his responsibilities broaden from weapons management to encompass tactical oversight, operational stability, and expanded command functions, reflecting his professional growth.

During this period, his conflict with father Kozo Nanbu intensifies. Kozo's significant influence within Earth's military economy through Nanbu Heavy Industry generates both professional and personal friction for Yasuo aboard the Asuka, remaining an ongoing element of his character development.