Mansaku Matsuura, nicknamed "Cabbage" for the Ariocarpus retusus cactus tattoo resembling a cabbage on his shoulder, is one of seven protagonists. Incarcerated at age 17 in Shōnan Special Reformatory for drunken violence and assault causing grave injuries stemming from underage drinking, he possesses an imposing 203 cm (6'8") stature. Despite this, he exhibits a naturally gentle, kind-hearted, and quiet disposition, often appearing the most cheerful and calm among his cellmates. His simple-minded nature includes a notable love for eating, which influences his early post-release work at a construction company partly for access to food.

During incarceration, Mansaku demonstrates unwavering loyalty and protective instincts. When Rokurouta Sakuragi is severely beaten by new cellmates instigated by a corrupt officer, Mansaku carefully tends to his wounds, wiping away blood clots. He rarely uses his immense physical strength aggressively, reserving it primarily to defend fellow inmates during reformatory abuses. His friendships, particularly with Tadayoshi "Soldier" Tooyama, become foundational to his identity.

Post-release, Mansaku initially struggles with direction due to his non-confrontational personality and lack of clear ambitions beyond culinary interests. His trajectory shifts when former gangster Aritou mentors him into professional wrestling. Adopting the ring name "Mammoth Matsuura", he builds a career in the United States. While wrestling abroad, he supports Mario Minakami's physical rehabilitation in Hawaii after a boxing-related hand injury, facilitating Mario's recovery through housing and encouragement.

In his personal life, Mansaku develops a connection with Ruriko, a third-generation Japanese American from Hawaii training in Japan. Though she initially expresses interest in Mario, Ruriko later recognizes Mansaku's steadiness and marries him. His story concludes with a stable marriage, continued involvement in wrestling, maintained ties with former cellmates, and modest personal contentment.