Kai Tōdō tutors Ichika Tachibana in mathematics and science. He is the twin brother of Sei Tōdō, sharing a close bond with him. Standing 185 cm tall with brown hair, brown eyes, and blood type A, Kai possesses a more extroverted, dynamic, and physically active personality than Sei, enjoying soccer in his free time. His given name "Kai" uses the characters for "painting" (繪) and "commission" (委), while the surname "Tōdō" combines "wisteria" (藤) and "temple" (堂).

Kai and Sei withhold crucial knowledge about the Djinn ritual from Ichika, aware due to Sei's prior participation six years earlier. Kai visibly disapproved when Sei gave Ichika the charm initiating the trials, betraying their foreknowledge of the danger. His protective streak flares when Sei intervenes in Ichika's ordeal and turns to stone as punishment. During the ritual's climax, Kai sacrifices the life energy sustaining his human form to revive Sei after his stone form shatters, reverting Kai to his original state as a mirror shard.

Kai harbors romantic feelings for Sei, informing his distress over Sei's ritual involvement and his ultimate self-sacrifice. In the OVA *Shotō no Futanatsu*, Kai's mirror shard temporarily regains human form alongside Manatsu on Christmas Eve, allowing a final reunion with Sei before both fragments return to Saya's mirror.