Marimo Bandō is a second-year student at Kameido High School and a member of the tennis club, known for extreme eccentricity, immaturity, and consistent lack of self-control. Her actions frequently become socially unacceptable or legally questionable, including wearing another girl's panties on her face, consuming undergarments, and attempting to kidnap a child, leading to police attention. Despite this, she possesses an underlying kind nature.

She repeatedly invades privacy, such as attempting to remove a clubmate's underwear or lifting another member's panties with a tennis racket. Such actions occasionally feature in official promotional artwork. During the club's first year, she grabbed a fellow member's breasts while claiming police pursuit without understanding the reason. She also exhibits bizarre habits like sleeping while standing upright with her head against a wall.

Marimo displays significant gaps in general knowledge and numerous misconceptions, often inadvertently endangering herself or others. Yet, when alone or without other club members, she occasionally serves as an unexpected voice of reason, demonstrating situational awareness despite her usual lack of judgment. She cohabits with Tomarin, an extraterrestrial who crash-landed on Earth, after they became roommates and friends. This living arrangement further enables their shared misunderstandings about Earthly norms.

Her fixation on undergarments is reflected in the fifth season opening theme "Qunka," which she performs; its lyrics explicitly reference smelling and consuming panties. Across all seasons, OVAs, and the spin-off *Usakame*, her core traits remain consistent without significant development or backstory expansion.