Kei Agemaki is a second lieutenant in the imperial army, assigned as the human representative to the Ministry of Spirit Affairs. He possesses medium-length blond hair swept to the left and golden-greenish eyes, noted for his prince-like handsomeness.

Agemaki harbors a severe fear of spirits and heights rooted in childhood trauma. His father, a renowned general, harshly berated him for expressing fear of spirits. This phobia deepened after his cat Itsue vanished following a spirit encounter, leading him to blame spirits for the loss. His mother and sister possess spiritual awareness, perceiving spirits invisible to most humans and recognizing Zakuro's half-spirit nature despite her concealed ears.

Initially, his fear creates friction with his spirit partner Zakuro, who derisively calls him a "wimp" for needing her protection. His attempts at rapport often falter, like mistakenly offering Zakuro a candle to eat, unaware fox spirits avoid them. Despite this clumsiness, he demonstrates persistent kindness, repairing Zakuro's broken pendant—an emblem of her mother—showcasing his craftsmanship.

Agemaki undergoes substantial evolution. He gradually overcomes his phobia, evidenced by interacting with previously terrifying spirits like the pumpkin-headed Sakura and Kiri, developing a paternal rapport. His relationship with Zakuro shifts from mutual denial to romantic acknowledgment; he experiences jealousy over her interactions with others like Hanadate, confessing his feelings in a culminating kiss.

His interpersonal dynamics extend beyond Zakuro. He shares a friendly, occasionally misinterpreted bond with Susukihotaru, collaborating during crises like mutual kidnappings. With fellow lieutenant Riken Yoshinokazura, he maintains strong camaraderie, frequently training together and discussing missions.

While lacking supernatural abilities, Agemaki is proficient with a sword, briefly holding his own against the half-spirit Daidai in combat. His role emphasizes human resilience and growth.