Masu Tsunashima, known as Masu-san, is the cook aboard the starship Arcadia. She appears as an elderly woman with an ovaloid body shape, gray hair secured in a bun, circular glasses perched low near her mouth, and eyes set high on her forehead. Her standard outfit comprises a shirt, vest, skirt, apron, and practical shoes.

Her personality blends cheerfulness with deep dedication to her culinary responsibilities. She treasures the crew's camaraderie but reacts with fierce protectiveness over her kitchen domain, wielding knives against anyone—especially Bird or Me-kun—who tries to steal ingredients, highlighting her commitment.

Significant personal history shapes her past. Engaged to Gozo Otowara, circumstances thwarted their marriage, leaving her believing she was abandoned. Years later, she discovered the truth behind their separation and briefly reunited with Gozo. This reunion ended tragically when Mazone aliens killed him. In the loss's aftermath, crewmember Miime provided her emotional support.

A longstanding professional dynamic exists between Masu-san and the ship's medical officer, Dr. Zero. Their interactions often spark comedic conflict due to his unauthorized raids into her kitchen to procure sake. Beyond these disputes, they share a deeper history of collaboration, having once worked together to aid the Tokaga people, adding complexity to her place within the crew's social structure.