Matsu operates as Sabu's subordinate within the Menso yakuza. His duties involve executing Sabu's orders, though the narrative explicitly questions his competence for the violent, high-stakes yakuza existence; he displays uncertainty about escaping this life.

He is romantically involved with Kayo, a sex worker. This relationship gains significance when Kayo becomes pregnant. Matsu's devotion manifests violently when members of the rival Tengenji gang insult Kayo during an assault on him. Provoked by their physical attack and verbal abuse of Kayo, Matsu retrieves a gun Sabu had entrusted to him for safekeeping. His impulsive use of the firearm against his attackers marks a critical escalation. This act risks sparking a full-scale gang war between Menso and Tengenji due to the attack's severity.

Matsu's decision forces Sabu into immediate action to manage the fallout, striving to prevent further bloodshed. This sequence highlights Matsu's tendency for rash decisions driven by personal emotion, particularly his protectiveness towards Kayo, rather than strategic thinking aligned with yakuza protocols. His actions heighten the danger for both Sabu and Koiko as intergang tensions intensify. The narrative positions Matsu's involvement in the conflict as stemming from his unsuitability for his role and his reactive, poorly considered responses to personal provocations.