Matsu is a middle-aged man who operates a snack bar as a public front, but this cheerful establishment masks his true activities as a deep-seated figure in the criminal underworld. He maintains strong ties with the Makuro group, engaging in a wide range of illicit operations including weapons trading, information brokering, corpse disposal, and the smuggling of rare animals. His appearance is distinctly marked by the absence of his left eye, a permanent scar from his violent past that gives him a unique and weathered look. His real name is initially unknown, and he is referred to simply as Matsu, a name given to him by the man who controls him.

In terms of personality, Matsu typically presents a cheerful and lively demeanor, greeting others with a bright smile and a memorable, unique way of speaking. However, this outward friendliness is a carefully maintained facade. Beneath the surface, he is an extremely cautious and calculating individual, a trait born from years of navigating the dangers of the criminal world. He is known for being thoughtful and deliberate in his actions, generally preferring to avoid unnecessary trouble when possible. Despite his jovial exterior, he carries a deep, underlying worry for his family, which is the primary driver of his actions.

Matsu's background reveals a past life as a skilled hitman with notable proficiency in handling firearms. His path crossed with Yamaoka, a powerful figure who, during a past confrontation, took Matsu's left eye. To ensure Matsu's complete subservience, Yamaoka took his daughter hostage, forcing him to work as a personal subordinate and carry out numerous illegal jobs against his will. This hostage situation is the central motivation for his criminal activities; his primary goal is not wealth or power, but the protection and safety of his daughter, who is a college student. He is trapped in a life of crime he wishes to escape, dreaming of a normal existence.

Within the story, Matsu serves as a conflicted underworld figure who is reluctantly drawn into major conflicts. He is forced to cooperate with Sunakawa by providing information about the protagonist, Fable, which further entangles him in Yamaoka's schemes, including participation in the assassination of Hamada. His role is that of a man caught between his desire for freedom and the chains of his past. His key relationship is with his captor, Yamaoka, who holds absolute power over him through his daughter. He also develops a crucial alliance with the Satou siblings, particularly Youko Satou, who helps him at a critical moment.

Matsu undergoes a significant development arc, marked by his decision to finally break away from Yamaoka and retire from the underworld. Seeking help from the Satou siblings, he agrees to a condition set by the crime boss Ebihara: he would be forgiven for the murder of Hamada if he could turn over custody of an individual named Jiro. His attempt to capture Jiro fails, forcing him to shoot the man. Believing he will be executed for this failure, he is instead granted a reprieve. Through the mediation of Youko Satou, Ebihara forgives Matsu, offering him a second chance. He finally reveals his real name as Yamaoka and leaves the criminal world behind, returning to his daughter to live as a civilian.

Notable abilities include his expert skill with firearms, a talent honed during his time as an assassin that remains formidable even with the use of only one eye. Beyond combat, he is highly capable in information gathering, keeping secrets, and managing both legal and illegal business operations with a shrewd and experienced hand.