Koyori Tōdō serves as the middle sibling among the three Tōdō sisters, positioned between eldest Tsumugi and youngest Kasane. She resides in Yamabiko, a rural town, and attends school as a first-year student. Her family connections include hospital patient Keita Yamakawa, whom she visits, and relative Gengoro Kamakawa, known as Grandpa Gen.

Physically, Koyori stands at 1.46 meters, ranking among the shortest main characters. Her light green hair falls in shoulder-length pigtails secured by dark green or black ribbons, complemented by brown-orange eyes. She frequently appears in her school uniform: a peach-colored shirt, brown tie, red skirt, and knee-high brown socks.

Energetic and tenacious, Koyori exhibits a strong-willed, outspoken personality. She initially displays hostility toward Kazuma Hoshino after mistakenly blaming him for another's distress, pelting him with acorns and shadowing him suspiciously. Her perspective shifts when he cushions her fall from a tree, revealing his kindness. She operates by a self-imposed hero code—helping those in need while instructing recipients to squat and close their eyes before she vanishes with a "poof" sound effect. Despite her boldness, romantic topics trigger childlike embarrassment, causing blushes and awkward reactions.

During the Hikonan Race, Koyori wears a bridal costume while Kazuma dons a groom's outfit. After spraining her ankle mid-event, Kazuma carries her across the finish line to secure her victory. Her motivation for competing stems from uplifting Keita Yamakawa before his operation. Though she shares a teasing dynamic with Kazuma, no romantic relationship develops between them.

Beyond family, her social circle includes friends Ui Nakatsugawa, Ibuki Hinata, Madoka Kōmoto, Hina Sakai, Ayumu Hoshino, and Daigo Minamikokubaru. These bonds thrive through Yamabiko's school and community interactions.