Koharu Tanaka begins as a hesitant first-year high school student recruited into the unofficial Tesaguri club, where members brainstorm alternative activities for school clubs by subverting common anime tropes and stereotypes. Fellow members affectionately nickname her "Koharun." Raised primarily by her grandmother, she exhibits outdated cultural tastes that cause occasional confusion in modern pop culture discussions, cementing her role as the group's "tsukkomi" due to her reactions and demeanor.

Over time, Koharu evolves from uncertainty to active participation, devising absurd club concepts like drama performances requiring random audience members to ad-lib roles or scavenger hunts with bizarre item requirements. Her idiosyncratic perspective surfaces in proposals such as combining kendo with nudity under consent-based conditions or integrating warm undergarments into sports. She frequently becomes the target of light teasing, particularly about her reserved nature—exemplified when embarrassing secretly-taken photos feature in club karuta games. Her reactions heighten meta-humor, like distress upon realizing her internal monologues are audible during fourth-wall-breaking moments.

Koharu engages in meta-narrative discussions, collaborating to justify alternate timelines where graduation is avoided to preserve the club. She participates in narrative resets, like manipulating a stopwatch in the opening sequence to halt time symbolically. In a third-season collaboration spin-off with "Minarai Megami: Puru Purun Charm," she interacts with its characters, confirming her ongoing club involvement. Her growing confidence emerges when she coins the nonsensical catchphrase "DONCHIKI DONCHIKI," spawning a recurring gag and the secondary nickname "Donchiki Tanaka."