Karin Sasamori founded and presides over the "Mystery Club," channeling its mission into investigating occult phenomena, supernatural events, and unsolved mysteries rather than conventional detective fiction. Her appearance is striking: golden-orange hair styled into spiky twin pigtails accented by a pumpkin-shaped hairpiece, bright blue eyes, and a slender frame often clad in a sailor uniform with a pleated skirt and knee-high socks. Eccentric and hyperactive, her obsession with the occult surfaces in quirks like reciting eerie egg-themed nursery rhymes and zealously consuming egg sandwiches—idiosyncrasies that unsettle peers.

Determined to keep her club alive, she blackmails the protagonist into joining as its second member, revealing her ruthless resolve to defy societal dismissal. This manipulative streak extends to memory-altering narratives, where she fabricates romantic history with the protagonist by rewriting his recollections, blending opportunism with relentless persistence.

Her interactions brim with mischievous antagonism, exemplified by assigning nicknames like "Yuma-chin" to incite reactions. During school festivals, she engineers chaos through tactics like doctored videos to stage compromising scenarios, showcasing her flair for theatrical deception. While spin-offs such as *AnotherDays* relegate her to a supporting role, her core traits remain static, lacking substantial growth across official stories. Relationships with peers orbit club-related antics or humorous misunderstandings, never delving into emotional depth.