Miki Onimaru is the protagonist of the series and works as the delivery girl and poster girl for her family's Chinese ramen restaurant, commonly referred to simply as Onimaru. Though she is twenty years old, she exhibits the mental and emotional maturity of a twelve-year-old, which is reflected in her boundless energy, short attention span, and intensely competitive nature. She has a very hot temper and is easily distracted; while making deliveries, she frequently stops to help stray dogs, chase insects, or fight bit-part troublemakers. Her personality is boisterous and active, and she often initiates fights or competitions seemingly on impulse. Despite her violent tendencies, Miki is genuinely caring and kind-hearted, always willing to assist anyone in need. This gentler side becomes especially prominent when she is intoxicated, at which point she transforms into a shy, polite, and conventionally ladylike version of herself.

Miki's primary motivation is not deeply explored; she appears driven by a simple desire to help others and a love for physical confrontation. Her role in the story is that of an unstoppable force of chaos who both creates and resolves conflicts within her neighborhood. She is the linchpin of the restaurant's popularity, as customers often come specifically to watch her brawl with her mother, Makiko Onimaru, who runs the shop with an iron fist. Key relationships include her rivalry with Megumi Kannazuki, a fellow poster girl from a nearby bakery; Kankuro Nishiyama, a young man who constantly challenges her to avenge past bullying but never succeeds; and her mother, who is one of the few people who can physically overpower her. Other acquaintances, such as the gentle giant Toshiyuki and the star-struck Akihiko Ohta, orbit her chaotic adventures.

Miki undergoes little character development; the series is a slapstick comedy in which her personality remains largely static. Her notable abilities include superhuman physical strength, a frantic and unpredictable fighting style, and a remarkable capacity to turn even mundane tasks into over-the-top battles. She is famously inept at cooking, dishwashing, and any job that requires focus, yet her combat prowess makes her an unofficial protector of the local shopping district.