Aga Serene, a supporting character and rival to protagonist Nabu Ancient within the Yamato 2520 narrative, lives on the frontier planet Rinbos in Globular Cluster M27 under Seiren Federation military occupation. He competes with Nabu in air races, actions which inadvertently activate planetary security systems and draw Seiren attention. Their rivalry escalates into personal conflicts and physical confrontations, including disputes over romantic interests like Marcie Shima, establishing Aga as an initial antagonist in Nabu's circle.

Unlike peers resisting the Seiren, Aga actively collaborates with the occupying forces. He works inside their domed city, constructed over the ruins of Osaka, and seeks entry into the Seiren military academy. This ambition creates friction with other Rinbos inhabitants who perceive his alignment as a betrayal.

Aga's loyalty shifts dramatically after Nabu discovers a data disk from the 17th Space Battleship Yamato. The disk's schematics enable the construction of a new vessel, the Yamato 18th, in an abandoned Seiren shipyard. Aga joins Nabu and other youths in this project, marking a pivotal turn from adversary to ally. His technical skills and knowledge of Seiren operations contribute directly to completing the Yamato and the crew's subsequent escape from Rinbos.

As part of the Yamato's crew, Aga participates in confrontations against both Earth Federation and Seiren forces reigniting hostilities over ancient technology. The series' discontinuation after three episodes leaves the narrative unresolved beyond these events.