Natsumi Mizuki is a young high school student who becomes intertwined with the world of the GetBackers after hiring them for a retrieval job. She is sixteen years old and works part-time as a waitress at the Honky Tonk cafe, where she is the sole employee. Her initial encounter with Ban and Ginji occurs when a small cat-like toy, a precious memento made by her late mother before she started high school, is stolen. After successfully recovering it, Natsumi becomes a close friend of the duo and a regular fixture at the cafe, where she also helps manage the debt the GetBackers owe to the establishment's owner, Paul Wan.

Natsumi possesses a cheerful, youthful, and bright personality that often borders on naivety. She is deeply devoted to Ban and Ginji, frequently worrying about their safety during dangerous missions and staying up late at the Honky Tonk to welcome them back. This loyalty is a central aspect of her character, as she acts as an unofficial supporter and groupie for their retrieval business. She has a strong sense of responsibility and, despite her generally optimistic and comedic interactions with the cast, occasionally displays a maturity beyond her years, likely influenced by her time working alongside the pragmatic Paul.

Her motivation is rooted in a genuine care for her friends and a desire to contribute to their well-being. She promised Ban and Ginji that she would ensure they never went hungry, and she takes her self-appointed role as their caretaker seriously. She also seeks to prove herself capable, taking on a retrieval mission on her own when the GetBackers are occupied with another case, showing a brave, if somewhat reckless, initiative.

In the story, Natsumi serves as an anchor to the ordinary world for the main characters, providing a warm, stable, and nurturing presence at the Honky Tonk. While she is not a fighter and lacks formal combat abilities, she has standout talents in table tennis, shogi, and cooking. Her notable abilities are these specific, non-combat skills, particularly her amazing ping-pong proficiency. Her relationships are key to her role: she shares a close, supportive friendship with Ban and Ginji, a respectful working relationship with Paul Wan, and a more flirtatious dynamic with Ginji in the manga. In the manga, her background differs from the anime; she is the daughter of an eminent virus researcher and her first case with the GetBackers involved rescuing her genius dachshund named Lucky, a storyline that also introduced the character Hevn to the series chronologically. Over the course of the series, Natsumi grows from a client into a steadfast and integral member of the extended support network around the GetBackers, her development marked by her increasing confidence and her unwavering, quietly courageous loyalty.