Masatsuki Nenga is a junior Galactic Patrol officer partnered with his older sister, Mutsuki. Their primary mission involves capturing the space pirate Mito, motivated by a need to restore honor to their family name following their father's disgrace within the Patrol. Initially overconfident and comically inept, Masatsuki deploys a device called Kaizou-kun to transform ordinary objects into combat mecha during pursuits, though these efforts invariably fail against Mito's crew. His species biology compels him to strictly avoid water, a weakness sometimes exploited in confrontations.

Masatsuki's psyche fractures upon witnessing the corrupt Galactic Patrol leadership authorize fire on civilians. This plunges him into mental collapse, marked by obsessive declarations of "I AM JUSTICE!" and increasingly violent, erratic mission behavior. Following this crisis, he undergoes significant development, ultimately defecting from the Patrol. He allies with former enemies, including Mito and the newly crowned Queen Aoi, to confront the greater threat posed by the malevolent First Queen Hikari in later installments.

His complex relationship with Mutsuki remains a constant, occasionally manifesting possessiveness, particularly concerning her growing bond with Aoi. This dynamic persists even after their transition from foes to allies by the story's conclusion.