Masatsuki Nenga serves as a Galactic Patrol officer alongside his older sister, Mutsuki Nenga. Driven by their father's dishonorable discharge from the Patrol, both siblings relentlessly pursue high-profile captures like the space pirate Mito to restore their family's standing. As a nitrogen-breathing alien, Masatsuki shares his sister's lethal vulnerability to water and stress-induced mushroom growths.

He consistently overestimates his own capabilities, displaying an intense, fanatical obsession with justice. On missions, Masatsuki utilizes a device called Kaizou-kun to transform ordinary objects like construction vehicles and vending machines into combat mecha. His tactical approaches frequently prove ineffective, leading to repeated failures against Mito's crew, including an undercover operation where he impersonated Mutsuki using a full-body latex suit.

Masatsuki exhibits a notable sister complex, showing protectiveness toward Mutsuki and jealousy regarding her interactions with Aoi Mitsukuni, Mito's son. A critical turning point occurs when he witnesses the Galactic Patrol firing upon civilians. This triggers a psychological breakdown, causing him to question his moral foundations and leading to episodes where he screams "I AM JUSTICE!" while firing his weapon erratically.

During the second season, following Aoi Mitsukuni's ascension as galactic queen, Masatsuki undergoes a Heel–Face Turn by joining Mito's faction. His loyalty shifts to supporting the new regime as a court soldier, where he participates in challenges to Aoi's rule. This transition from antagonist to ally demonstrates adaptability, though his fundamental personality traits remain consistent. His development stems primarily from confronting the contradictions within his previously rigid concept of justice.