Keisuke is introduced as the younger half-brother of the protagonist, Keiji, and is a crested rooster who matches Keiji in strength. His background is closely tied to a troubled family history; his sudden appearance forces Keiji to confront his painful past, specifically his deteriorating relationship with their father following a family tragedy. Keisuke seeks out Keiji to enlist his help on a desperate rescue mission, as their father has been captured by a new and powerful class of enemies known as the kijin.

In terms of personality, Keisuke is depicted as more complicated and vulnerable on the inside compared to his stoic and brash brother, a trait that the series creator has noted as relatable. While Keiji is a wanderer who actively avoids attachments, Keisuke actively seeks out his brother to mend family bonds and confront a shared threat. His role in the story is therefore a crucial catalyst, shifting the narrative from Keiji's solitary revenge quest to a family-centered conflict. By claiming to be Keiji's brother, Keisuke expands the protagonist's family tree and brings the brothers' shared lineage and its mysterious connection to the monsters into focus.

Keisuke's arrival also serves to challenge Keiji directly. He demonstrates a level of power that can surpass his older brother, to the point of defeating him and wounding Keiji's pride as a fighter. His primary motivation is not revenge but rescue, as he seeks to save their father from the devils, a mission Keiji reluctantly agrees to join despite his resentments. Regarding his abilities, Keisuke is noted to be a formidable fighter on par with Keiji, possessing the strength to overcome him in battle.