Kotetsu Dōmyōji is a first-year student at Jokei University, possessing black hair and grey eyes. He shares a close, brotherly bond with Chihiro Kamina and lives with him following Chihiro's loss of his parents. A master of Dōmyōji-style sword fighting, Kotetsu has never defeated Chihiro, hindered by his preoccupation with frontal attacks. His personality is extremely straightforward, though he displays moments of delicacy and cowardice.

His life changes drastically during Tokyo's Great Collapse. After his father transforms into a monster and is lost, Kotetsu awakens blood-based powers, manifesting as a two-handed katana-like weapon formed from his blood. His father's death fuels a vengeful drive targeting Jun Aoi, whom he holds responsible for opening the gate leading to the tragedy.

Progressing through the story, Kotetsu fights alongside Chihiro as his partner, channeled grief into protective actions. He develops a connection with Tsubaki Manazuru, suggesting a gradual shift from his singular focus on vengeance. His combat style evolves beyond straightforward attacks, allowing him to better support Chihiro in factional confrontations.