Ginji Matsuzaki serves as the acting wakagashira, or underboss, of the Washimine Group, a yakuza family in Tokyo. At the time of his introduction, he is no longer an active gangster but works peacefully as a street stall vendor. This humble occupation masks a lethal past, as he was once a feared assassin known throughout the underworld as Hitokiri Ginji, or Manslayer Ginji. His nickname was earned through his deadly proficiency with a katana, a weapon he wields with extraordinary skill in a world increasingly dominated by firearms.

Physically, Ginji is an imposing figure of Japanese descent, standing at approximately 194 centimeters tall, or six feet four inches, with a broad and muscular build. He has short, spiked black hair with the sides of his head shaved, pale skin, and a short goatee. He is almost always seen wearing thin sunglasses, a large, dark green overcoat over a white shirt, tan pants, and open-toe sandals. Concealed beneath his clothing are extensive yakuza tattoos covering his back and arms, featuring traditional imagery like a gashadokuro, a samurai, flowers, and flames.

Ginji's personality is defined by a stark contrast between his peaceful aspirations and his violent capabilities. On the surface, he is a calm and serious individual with an imposing presence. He genuinely enjoys his simple life as a food vendor, suggesting a desire to leave his bloody history behind. However, when provoked or when his loyalties demand it, he displays a vicious, violent, and sadistic side that matches the most brutal fighters in the Black Lagoon world. He seems to harbor deep regret for his past as a murderer but demonstrates no hesitation or mercy when he decides to take up arms again.

His primary motivation is his profound sense of honor, duty, and personal loyalty, specifically toward Yukio Washimine, the young daughter of the group's late patriarch, Ryuzo Washimine. Ginji had protected Yukio since she was a child and held a deep wish for her to live a normal life, free from the criminal destiny of her family. For this reason, he retired from active yakuza life, refusing the orders of the acting head, Tsugio Bando, because he felt the group had begun engaging in dishonorable practices like prostitution and drug trafficking, which went against the code Ryuzo upheld. His primary motivation is reactivated not by a sense of clan duty, but by his personal devotion to Yukio. When she decides to take control of the group to avenge Bando's murder, Ginji bows to her and unhesitatingly offers to become her protector and assassin, sacrificing his peaceful existence for her sake.

Within the story, Ginji is a central figure in the Fujiyama Gangsta Paradise arc. He begins as a supporting character who reluctantly becomes Yukio's fiercest weapon in her war against the Russian mafia organization Hotel Moscow, led by Balalaika. His key relationship is, without a doubt, with Yukio Washimine. He is her most loyal and humble bodyguard, and his entire role is structured around protecting her and fulfilling her will, even when he believes her path will lead to tragedy. He also shares a notable, if brief, relationship with Revy. After a violent first encounter, they form an uneasy and silent truce to rescue Yukio from the traitorous yakuza Chaka. They communicate not through language but through a mutual recognition of each other as ruthless killers. Revy nicknames him Jumbo due to his large stature. In their duel, Ginji proves to be one of the very few opponents who can match Revy, even gaining the upper hand when he shatters her pistol and stabs her knee; he would have likely killed her if not for being distracted by Yukio. Ginji's other significant tie is to the late patriarch Ryuzo Washimine, whom he deeply respected and fought for, and to Tsugio Bando, with whom he has a complicated relationship of mutual respect but differing views on honor.

Ginji's character undergoes a tragic but linear development from a man at peace to a warrior consumed by a lost cause. He begins as a retired killer content with his life as a street vendor. He is forced to confront his past and, driven by his loyalty to Yukio, willingly transforms back into the Manslayer Ginji. He does not evolve into a different person but fully embraces his old identity for the sake of another. The arc concludes with his death at Revy's hands in a prearranged duel. After realizing that their war against Balalaika is unwinnable, he and Yukio accept their fate, and his death is a final, honorable act of service.

Ginji Matsuzaki's most notable ability is his preternatural mastery of kenjutsu, the art of swordsmanship. His skills are so exceptional that they appear almost superhuman. His signature feat is the ability to cleave bullets in two in mid-flight with his katana. Coupled with this is his incredible speed and agility, which allow him to dodge gunfire from multiple foes simultaneously. Because of this, he almost completely eschews the use of firearms, relying solely on his blade. His physical strength and precision are such that he can not only cut bullets but also slice an opponent's handgun in half and sever their limbs in a single fluid motion. In essence, he is a living anachronism, a master samurai in a modern world of automatic weapons, whose skill is so immense that it overcomes the perceived technological disadvantage.