Mami Endo has light brown eyes and brown hair styled in a semi-bobcut with a trimmed fringe. She wears Hakuoh Academy's standard uniform during school hours and a maid outfit while working at the Lamp House café. Her personality blends friendliness, supportiveness, and carefree helpfulness toward friends, balanced by a consistent level-headedness. She has been close friends with Marika Kato since at least middle school, shares shifts at the Lamp House, and states she has never traveled to space.

During the Recruitment Arc, Mami discusses space pirates and their school's replacement teacher with Marika after Marika learns of her pirate heritage. At the Lamp House, she observes intimidating customers focused on Marika, serves Chiaki Kurihara, and witnesses a café gunfight, later expressing relief at Marika's safety. She voices concerns about their friendship amid Marika's new challenges during a walk home; Marika reassures her, valuing Mami's composure. After exams, she accompanies Marika to the beach, learns of her decision to captain the Bentenmaru, and holds Marika's belongings when she leaves for Letter of Marque paperwork.

In the Golden Ghost Ship Arc, Mami wakes Marika in homeroom to relay a teacher's message. At work, she serves Chiaki after Marika departs abruptly, later walking with Chiaki to discuss Marika's tendency to overextend. When Gruier Serenity transfers to Hakuoh, Mami asks Marika if she has seen the princess and informs Gruier about the yacht club and attendance-faking methods during Gruier's Lamp House visit. Post-Marika's return, Mami modifies Marika's pirate uniform, suggesting she wear it for recruitment. She questions Marika about yacht club progress, mentions her knitting club's success, re-proposes the uniform idea—which Marika declines—and leaves Gruier and Marika to close the café.

Throughout the Hakuoh Pirates Arc, Mami brings Marika ribbons and flowers to enhance her uniform, noting the Bentenmaru's unusual silence. Before the Odette II departs, she provides Gruier with handmade costumes. Working at the Lamp House, she declines joining the yacht club, stating the port suffices for her. She serves Ririka Kato, discussing career shifts and agreeing Marika's efforts inspire others. Later, she watches Marika's televised broadcast hijacking, discusses the events with her, and accepts thanks for the costumes.

In the anime-exclusive Nebula Cup Arc, Mami lunches with Marika, commenting on her sunbathing, and encounters exhausted yacht club members at the beach. During the Pirate Hunter Arc, she handles Marika's work notifications during pirate absences, noting Marika's disappearance would feel unnatural. She greets Chiaki cheerfully after her Hakuoh transfer, serves parfaits to Marika, Chiaki, and Gruier, and lends Gruier a maid outfit. Using a pirate password, she arrives at Marika's home with Grunhilde. While Marika attends a pirate council, Mami lunches with others, hoping for her success. She welcomes Marika back post-Grand Cross victory, jokingly calling her "Captain Kato," and passes final exams.

In the film *Abyss of Hyperspace*, Mami works diligently at the Lamp House amid rival restaurant competition, designing a mascot costume in response. She serves Marika and Kanata Mugen, greets a disheveled Chiaki while mimicking her retort, and gifts Marika a new uniform she designed. She steps aside during Marika's confrontation with Scarlett Cypher.

Mami applies tailoring skills to modify Marika's pirate uniform, design yacht club costumes, create the Lamp House mascot, and craft Marika's new film uniform. As a Hakuoh knitting club member, she supports Marika steadfastly, maintains friendliness toward Chiaki (despite occasionally irritating her), and assists Gruier with information and outfits. She aids the yacht club through costumes but prefers not to venture into space.