Marimo Bandō is a 16-year-old high school sophomore and tennis club member, standing 169 cm tall and weighing 58 kg. Her defining trait is a collection of persistent, significant misconceptions about the world, frequently driving her to unconventional and problematic actions. These misunderstandings cause harm or trouble for herself and others, manifesting in behaviors like consuming girls' underwear or involvement in child kidnappings. Despite these tendencies, she possesses an underlying kind nature, coupled with immaturity and poor self-control.

Her misconceptions extend to interactions, notably with Tomarin, an alien whose spaceship crash-landed on Earth. Marimo becomes Tomarin's roommate, actively sharing her flawed understanding of Earth and compounding confusion through their exchanges, which fuels further chaotic situations.

While often the source of trouble, Marimo surprisingly becomes the voice of reason when separated from her fellow tennis club members, Yuri Oshimoto, Kanae Shinjō, and Nasuno Takamiya. In scenarios where the others engage in even more erratic behavior, her typically poor judgment gives way to grounded perspectives.

Her core traits and misunderstandings remain consistent throughout the series, multiple seasons, OVAs, and spin-offs, with no substantial character development. Her background, including family, upbringing, or life outside school, remains unexplored. Her portrayal consistently focuses on the consequences of her misconceptions within episodic comedic situations. Her relationships within the tennis club stay static, maintaining dynamic interactions with Yuri, Kanae, and Nasuno that consistently highlight her role in generating chaos through misguided actions, balanced by moments revealing her fundamentally well-intentioned nature.