Gusion Masaru ranks among Solomon's 72 demons, embodying Sloth. He shifts between two distinct forms: a small winged flying monkey in his chibi state and a massive, imposing giant monkey in his Hell manifestation.

He contracts with human Kōtarō Dōchin, who sacrifices cherished memories of his grandfather to forge their bond. This pact positions Gusion as Kōtarō's instrument, frequently exploited for personal gain or crisis resolution. Kōtarō exhibits an abrasive, angry personality, often directing sexual harassment and hostility toward others—including Gusion. Their relationship remains strictly transactional and antagonistic, defined by mutual exploitation for targeted goals rather than camaraderie.

Gusion wields the power Forgetfulness, enabling him to devour individuals' memories—particularly treasured ones—and later regurgitate them to their owners. This ability falters against the detective Akutabe, whose inherent power blocks memory consumption.

Across official media, Gusion repeatedly aids Kōtarō's schemes involving memory manipulation. In OVA 4, Kōtarō commands him to consume traumatic memories from Osanai, reversing psychological damage from a horrific event. When Osanai later refuses a demand, Kōtarō forces Gusion to regurgitate the memories as coercion. The same OVA depicts Kōtarō's failed attempt to use this power against Akutabe; Gusion is violently repelled and injured, underscoring both Akutabe's immunity and the demon's abusive servitude.

No details surface regarding Gusion's history among Solomon's demons before contracting with Kōtarō. His role remains static: he executes memory-related tasks under Kōtarō's orders without personal growth, evolving motives, or shifts in their dynamic. He primarily facilitates plot points centered on memory alteration within Kōtarō's narratives.