Koyori Tōdō is a first-year student at Yamabiko South Academy, the middle sister positioned between eldest Tsumugi and youngest Kasane. Standing 146 cm tall, she is the main cast's shortest member aside from Kasane. Her appearance features light lime-green hair styled in pigtails secured with black ribbons and distinctive orange eyes.

She initially exudes a headstrong, tenacious, and often loud personality, directing particular hostility toward protagonist Kazuma Hoshino due to rumors about his accidental kiss with Ui Nakatsugawa. Viewing him as disruptive, she actively monitors his actions, reflecting her protective and judgmental nature.

A pivotal shift occurs when Kazuma cushions her fall from a tree, revealing his kindness and prompting her to reassess him, leading to warmer interactions. Despite her spitfire attitude, Koyori possesses strong empathy and duty. She frequently assists others, including humans and animals, adopting a self-styled "superhero" persona where she demands crouching with eyes closed before theatrically departing with a "poof" sound effect, showcasing her childlike enthusiasm and desire to appear capable.

Her character development intensifies during the Hikonan Race triathlon, where she competes fiercely to fulfill a promise to Keita, the hospitalized grandson of a local widower, aiming to boost the boy's morale before his surgery. After spraining her ankle mid-race while wearing a wedding gown costume, Kazuma carries her to the finish line, securing her victory. This act solidifies her trust in him and underscores her determination to honor commitments despite physical setbacks.

Koyori exhibits pronounced romantic awkwardness, blushing intensely and struggling to respond to such topics, highlighting her emotional inexperience and contrasting her usual confidence. Her relationships extend beyond Kazuma; she shares a close bond with her sisters and acts as a supportive figure for community members like Gengorō Yamakawa and his grandson.