One day, while casually hanging out at home, Mina jokingly read a page about demonic invocations and accidentally summoned Kuroo, a demon from the Ring of Lust. Now, only Mina can see and hear Kuroo; he enjoys spending time with her and telling her about life in Hell. Humans who summon demons and become bound to them are known as "Gateways," and they allow the demon to see the mortal world. Kuroo is now Mina's demon, and she is his Gateway. Mina must now live alongside Kuroo. She can summon him to talk by saying, "Kuroo, break the veil." As a demon of lust, Kuroo can grant Mina's requests and desires in exchange for watching her have sex, having sex with her, or watching her masturbate, among other things. Kuroo is indifferent to human life and doesn't really understand humans. He often explains the infernal hierarchy, the nature of a "Gateway," and his past adventures with previous Gateways—all while enjoying Mina's reactions.