Mutsu, daughter of the Chidori space pirate group's president, grew up entrenched in slave trading operations. Forced into becoming vice-commander of Chidori's second division at age 14, she encountered a young Umibouzu during her interstellar travels; his tales of alien hunting fostered her appreciation for his experiences and past. When Chidori captured the adrift Sakamoto Tatsuma, Mutsu absorbed "business" concepts from him during his imprisonment. After her father's death, Chidori leadership plotted to execute Mutsu and their slaves. She planned self-sacrifice to aid the slaves' escape, but Sakamoto and the slaves overthrew the pirates, commandeered their vessels, and founded the merchant fleet Kaientai. Mutsu served as Sakamoto's second-in-command, managing daily operations due to his frequent absences and eccentricity. Demonstrating strategic acumen and combat skill, Mutsu countered crises like the flawed Renho treaty Sakamoto negotiated with Dark Vader, which endangered Earth. She identified the deception, coordinated Kaientai's anti-invasion efforts, and endured capture during diversionary talks. After escaping, she deployed Kaientai's secretly modified ships—transformed into the giant combat robot "Kaien" without Sakamoto's knowledge—to obliterate the Renho threat. Confronting resurgent Chidori pirates who captured Sakamoto, Mutsu eliminated former subordinates while feigning defeat, then swiftly defeated remaining foes. She revealed her Yato clan heritage to Gintoki during the clash, using him and Sakamoto as projectiles to destroy escaping pirates, showcasing her physical strength and pragmatism. Mutsu participated in pivotal conflicts like the Rakuyou Decisive Battle, where Kaientai exploited intelligence leaks to rescue Kawakami Bansai and aid Takasugi Shinsuke's survival. Post-battle, she reunited with Umibouzu in a medical facility, expressing desire to hear more of his stories. During the Silver Soul Arc, she accompanied Sakamoto to negotiate with the Altana Liberation Army. After Prince Enshou usurped control and activated the planet-destroying Hinokagutsushi, Mutsu was imprisoned but freed by defecting Amenotori staff. She intercepted a beam sword strike aimed at Sakamoto in the ensuing battle against Enshou, suffering severe injuries that removed her from combat but surviving the Amenotori ship's crash into Earth. Two years post-war, Sakamoto's cryptocurrency losses forced Kaientai's temporary dissolution. Mutsu engaged in black market activities, including delivering illicit goods like a weapon to Shimura Shinpachi. As a Yato, Mutsu has fair skin sensitive to sunlight, requiring constant use of a sedge hat. Her appearance features brown eyes and light brown straight hair tied loosely back. She wears a blue suit with a red scarf, purple cape, and black boots with pants tucked in. In youth, she had short hair and dressed in a cheongsam-style top with miniskirt and long coat. Mutsu maintains a calm, collected, and serious demeanor, contrasting Sakamoto's impulsiveness. This divergence sparks visible frustration, such as when Sakamoto neglected Renho crisis planning to play Uno. Despite differences, she exhibits unwavering loyalty to Sakamoto and Kaientai, embodying his "Never let your eyes off" philosophy regarding opportunities or threats. Her tactical decisions prioritize mission success over emotion, exemplified by preparing to fire on a sandworm trapping Sakamoto. Yet protective instincts surface in critical moments, like shielding Sakamoto from Enshou's attack. The character draws historical inspiration from Count Mutsu Munemitsu, a Meiji-era statesman who collaborated with Sakamoto Ryōma's real-life Kaientai to challenge the Tokugawa shogunate.