Helmeppo debuted as the spoiled son of Marine Captain "Axe-Hand" Morgan, wielding his father's authority to threaten and bully civilians in Shells Town. Exhibiting cowardice, he relied entirely on Morgan's protection while displaying arrogance and entitlement. His actions led to Roronoa Zoro's arrest after Zoro killed Helmeppo's pet wolf, Soro, for attacking a girl named Rika. Helmeppo broke his promise to free Zoro after a month of starvation, ordering execution after only nine days. This cruelty prompted Monkey D. Luffy's intervention, resulting in Morgan's defeat and Helmeppo's demotion to a Marine chore boy. Following Morgan's arrest, Helmeppo worked alongside Koby, harboring initial resentment. His perspective shifted when Morgan escaped and took him hostage. During this event, recognizing his father's brutality, Helmeppo disowned Morgan and chose to swim back to the Marines, marking the start of his redemption. Vice Admiral Monkey D. Garp, impressed by Helmeppo and Koby's humility, took them to Marine Headquarters for rigorous training. Under Garp's mentorship, Helmeppo transformed physically and mentally, gaining muscular strength, greater height, and growing longer hair tied in a ponytail. His combat skills improved, particularly with dual kukri swords, though he remained far below elite fighters like Zoro. He cultivated discipline, humility, and a sense of justice, shedding his former cowardice and entitlement. Rising through the ranks from chore boy to Chief Petty Officer and eventually Lieutenant Commander, he formed a close partnership with Koby, often providing levelheaded analysis to counterbalance Koby's aggression. Helmeppo participated in major Marine operations, witnessing Admiral Akainu's brutality and supporting Koby during the Paramount War at Marineford. He later assisted in arresting Boa Hancock after the Warlord system's dissolution, though temporarily turned to stone by her powers. As a member of the covert unit SWORD, he gained operational independence while remaining its lowest-ranked known member. His loyalty to Koby was evident during the Egghead arc when he shielded Koby from cannon fire during Garp's raid on Hachinosu to rescue him from the Blackbeard Pirates, demonstrating enhanced durability and swordsmanship. In *One Piece: Stampede*, Helmeppo joined the Marine response during the Pirates Festival on Delta Island. He fought alongside Koby, Hina, and other Marines against the assembled pirates, participating in the large-scale battle against Douglas Bullet's awakened Devil Fruit powers and the ensuing Marine Buster Call, contributing to containment efforts.