Karina Shiro is a police investigator dedicated to cases involving supernatural phenomena. He routinely partners with members of the Kawahira family to resolve incidents concerning spirits and demons. Actively collecting perilous mystical artifacts called "devil's toys," items crafted by the ancient wizard Sekidōsai, is another facet of his work. Karina wields the enchanted Angel Blade in his investigations and combat, employing signature techniques like "Holy Crush."

Sekidōsai is Karina's direct ancestor, and while he bears a strong physical resemblance to the ancient wizard, their personalities sharply contrast. Sekidōsai displays overt perversion, whereas Karina conducts himself with moral uprightness and professionalism as an investigator. However, narrative elements suggest Karina might harbor suppressed subconscious desires or perverse tendencies, occasionally surfacing through misinterpreted wishes or unintended situations, though never reaching the extremity of his ancestor's traits.

Karina occasionally provides legal assistance to Keita Kawahira when Keita faces arrest for public indecency, often a result of Yoko's retaliatory teleportation pranks. This dynamic highlights Karina's role as a liaison between conventional law enforcement and supernatural entities.

During the film narrative, Karina storms out of a party at Keita's apartment and later dies under mysterious circumstances. While initially attributed to a fatal fall, the absence of visible injuries casts doubt on the true cause. His death coincides with the disappearance of a demonic artifact—a miniature house—he took with him upon leaving. This event triggers an investigation led by Kaoru Kawahira and his Inukami, who suspect a connection between Karina's death, the missing artifact, and a shadowy initiative known as the "K-Project."