Masatsuki Nenga is a supporting character in the anime Space Pirate Mito. He is the younger brother of Mutsuki Nenga and serves as a Galactic Patrol officer alongside her. His primary motivation stems from a desire to restore the honor of his and Mutsuki's father, who was dishonorably discharged from the Galactic Patrol. This drive fuels his relentless pursuit of high-profile criminals, most notably the space pirate Mito and her crew.

Personality-wise, Masatsuki is characterized by an overestimation of his own capabilities. He frequently believes he can apprehend Mito, but his overconfidence often leads to failure. He displays an intense, almost fanatical obsession with justice, approaching his duties with a fervent dedication that can come across as naive or reckless. Despite his enthusiasm, he is a loyal team member who works closely with his sister, though his eagerness sometimes puts them in difficult situations.

As a nitrogen-breathing alien, Masatsuki shares a biological vulnerability with his sister: exposure to water is lethal to them, and stress can cause mushroom-like growths on their bodies. This unique physiology adds a layer of risk to their missions. In terms of role within the story, Masatsuki acts as a recurring antagonist, part of the galactic law enforcement repeatedly attempting to stop Mito's pirate activities. His relationship with Mutsuki is central to his character; they function as a sibling duo, with Masatsuki often deferring to her leadership while contributing his own zealous energy.

Throughout the series, Masatsuki remains a consistent presence in the chase after Mito, but his character sees little dramatic development. His abilities as a Galactic Patrol officer likely include piloting skills and combat training, though specific powers are not elaborated beyond his alien biology. He appears in both seasons of the anime, serving as a persistent if often outmatched obstacle for the protagonists.