Chikai Kuji, Toi Kuji's 25-year-old older brother born December 9, possesses messy dark hair and gray eyes, typically dressed in formal black attire. His worldview is rigidly defined by survival of the fittest, compelling him to dispose of anyone perceived as weak without hesitation. This philosophy fundamentally shapes his actions.

Following their parents' suicide due to overwhelming debt, Chikai assumed responsibility for Toi. To survive, he joined the Yurikamome yakuza gang, receiving a Tokarev pistol from its leader. Chikai later betrayed the gang, stealing their money and provoking the leader to target him and attempt Toi's kidnapping as leverage. The leader died by Toi's hand during this attempt. Despite initially caring for Toi and providing support, Chikai's "strong survive" ideology eventually led him to view Toi as weak. He promised to reunite with Toi after covering up the murder but ultimately broke this promise.

Chikai's interactions reflect his ruthless philosophy. He briefly bonded with Enta Jinnai over ice cream in an arcade, advising reconciliation with friends, unaware Enta referred to Toi. Later, when pursued by police and a katana-wielding yakuza, Chikai used Enta as a human shield to escape, showing no remorse and justifying it by his survivalist beliefs. In a critical confrontation, he demonstrated willingness to sacrifice even Toi, shooting at him after deeming him weak, despite Toi previously saving him from attackers.

His story unfolds through key episodes, starting with his separation from Toi in Asakusa as he left for business, warning Toi to avoid detection. Flashbacks reveal their happier family life before the debt-driven parental suicide and Chikai's subsequent descent into yakuza activities. Later episodes depict his ultimate betrayal of Toi, driven by his conviction that weakness justifies abandonment or elimination.