Garp

Monkey D. Garp is an esteemed and formidable Marine vice admiral who holds immense renown and power. He is the father of Monkey D. Dragon, the paternal grandfather of Monkey D. Luffy, and the adoptive grandfather of Portgas D. Ace. Garp played a pivotal role in Ace's birth and raised a young Luffy, whom he later entrusted to the Dadan Family. Garp is recognized as one of the key figures, along with Kong, Sengoku, Shiki, Silvers Rayleigh, Tsuru, and Whitebeard, from the time when Gol D. Roger, the "Pirate King," was alive. His notable deeds as a Marine soldier, particularly during the God Valley Incident, earned him immense notoriety and prestige, leading to his recognition as the "Hero of the Marines." Despite being offered the rank of Admiral several times, Garp declined the promotion each time.

Garp is widely acknowledged as a living legend and one of the strongest Marines ever. In his prime, he was renowned for repeatedly cornering Gol D. Roger, the Pirate King, in battle, and Roger himself attests to their near-fatal encounters. Garp's formidable power and immense popularity led to him being offered the rank of Admiral several times throughout his career, but he rejected it each time. Together with Roger, he fought and defeated the Rocks Pirates, one of the most powerful pirate crews in history, which included some of the most legendary pirates such as Edward Newgate, Charlotte Linlin, and Kaidou, as well as Roger's most formidable rival, Rocks D. Xebec. This remarkable achievement earned Garp the title of "Hero of the Marines." Further proof of his sheer strength and might is seen when Garp defeated Don Chinjao with a single punch, almost killing him. Don Chinjao's bounty was worth 542,000,000 beli in his prime, and he was said to be capable of splitting a continent with a headbutt. This feat earned Garp the epithet, "Garp the Fist." Chinjao himself admits that during that time, all pirates viewed Garp as a fearsome "Devil."

Despite being 76 years old, Garp displayed remarkable strength by easily defeating Marco the Phoenix, Whitebeard's second-in-command, which even Kizaru and Akainu struggled to do. During the War of Marineford, Garp was so enraged that he would have killed Akainu if Sengoku hadn't stopped him. When it was revealed that Luffy is Garp's grandson, it elevated Luffy's fame as even Demaro Black used it to impersonate him. Charlotte Linlin also acknowledged Garp's legacy when Luffy challenged her. Garp's rebellion and reputation are so great that even the World Nobles fear him. He possesses exceptional Haki skills, particularly in Busoshoku Haki, which he uses to injure Luffy, bypassing his rubber body. Garp also demonstrated his mastery of Kenbunshoku Haki, with enough expertise to mentor others, such as Koby.

Garp's Busoshoku Haki skills enabled him to counter and crush Chinjao's Busoushoku Haki-enhanced head drill over 30 years ago, showcasing his superior mastery compared to Chinjao, a formidable Busoshoku Haki user in his own right. At Marineford, Garp managed to injure Marco despite him being in his regenerative phoenix form. He is also capable of using Kenbunshoku Haki, with Dr. Fishbonen recommending Koby to seek Garp's expertise on the matter. Despite Doll's attempts to dissuade him, Garp sailed to G-14 and yelled for Helmeppo to join him in rescuing Koby from the Blackbeard Pirates.

Koby

Koby is a Marine Captain who, like Helmeppo, trained under Vice Admiral Garp. He is a member of SWORD, a special forces unit within the Marines. In his early days, he was a caretaker for the Alvida Pirates until he was rescued by Luffy, who later became his friend. Koby is known as a hero for his actions during the Rocky Port Incident and became a fully-fledged Marine soldier by the time of the Post-Enies Lobby Arc. In the Diary of Koby-Meppo Cover Page Serial, Koby and Helmeppo are the main characters. They are also the supporting antagonists of the Post-Enies Lobby Arc. During his initial appearance, Koby lacked any fighting skills and was only useful for his navigation abilities. However, his training under Garp and further development during the two-year timeskip has granted him immense physical strength, speed, and the ability to swim quickly. He has some knowledge of swordsmanship, which he practices with Helmeppo and Bogard.

Koby has risen to the rank of captain and is in charge of his own ship and lower-ranking Marines. He played a role in the Marines' attempt to arrest Boa Hancock, where he was one of the few Marines not turned to stone. However, he was captured by Marshall D. Teach and is currently a prisoner of the Blackbeard Pirates. While Koby possesses great strength and skill, he is no match for powerful foes like Boa Hancock or Marshall D. Teach. In Episode of Luffy, Koby demonstrated his impressive strength by shattering a massive metal disk with a single punch and easily executing Rokushiki techniques, including the Soru technique, which he learned from Vice Admiral Garp. Although he utilized Soru against Luffy twice, once in Water 7 and again at the Summit War of Marineford, it did not give him an advantage in either fight. In One Piece: Gigant Battle! 2 New World, Koby also demonstrated the ability to use Geppo and Rankyaku techniques. After the time-skip, Koby has honed his Rokushiki techniques to mastery level, even demonstrating a technique similar to Rankyaku using his arms in Episode of Luffy. He also awakened his Kenbunshoku Haki during the Summit War of Marineford, allowing him to sense the deaths of combatants around him. Since then, he has developed his Kenbunshoku Haki to an advanced level, as demonstrated by his ability to detect a torpedo heading towards a ship from a distance and sense Kyros's immense strength upon first meeting him.

Furthermore, Koby possesses the ability to use Busoshoku Haki. Following the Seven Warlords of the Sea system's abolition, Koby embarked on a mission to capture Boa Hancock in Amazon Lily. En route, he contacted X Drake through the Den Den Mushi, who informed him of the newly formed alliance between Kaido and Big Mom and CP0's arrival in Wano Country. With the aid of the new Seraphim, the Marines battled their way into Amazon Lily. However, when the Blackbeard Pirates also launched an invasion, Koby ordered S-Hawk to cease fire as it endangered the Marines. Despite Koby's demand that Hancock surrender to the Marines, the Empress refused and attacked, petrifying most of the Marines and pirates present. Teach took advantage of the situation and seized Hancock, neutralizing her powers. Although Koby attempted to negotiate Hancock's release, Teach refused to trust her, and Silvers Rayleigh intervened as a mediator. Eventually, Hancock was freed and restored the petrified individuals. The Marines and Blackbeard Pirates departed from the island, but news later surfaced that the Blackbeard Pirates had abducted Koby.

Helmeppo

Lieutenant Commander Helmeppo is a member of the Marines and the son of the former Marine Captain Axe-Hand Morgan. Prior to joining the military, he was a civilian who took advantage of his father's rank to get what he wanted. However, after undergoing rigorous training with Vice-Admiral Garp, he became a fully-fledged Marine and currently holds the rank of lieutenant commander. Helmeppo is also a member of SWORD, a secret group within the Marines. During the Romance Dawn Arc, Helmeppo's actions paint him as a supporting antagonist. However, he and Koby are the main characters of the Diary of Koby-Meppo Cover Page Serial. In the beginning, Helmeppo was weak and relied solely on his father's status to get by. After training under Garp, he gained exceptional physical abilities, as evidenced by his high jump while intercepting Zoro.

After the two-year timeskip, Helmeppo's skills had grown exponentially, allowing him to single-handedly defeat an entire pirate crew with ease. Helmeppo is particularly skilled in swordsmanship, having even forced Zoro on the defensive for a brief period. Post-timeskip, he has significantly improved his swordsmanship, even causing significant damage to a pirate submarine while underwater. Despite his interest in collecting weapons, including Zoro's Meito, Helmeppo possesses no real desire or ability to use any weapons apart from a single pistol.

Helmeppo, now a full-fledged Marine, carries a pair of kukri as his preferred weapons, which he keeps on his belt at the back of his waist. He first used them in a duel against Zoro in Water 7, but was easily defeated. Following the time-skip, he upgraded to a larger set of kukri. Helmeppo has suggested that he can use Kenbunshoku Haki, noting Koby's remarkable ability to sense an underwater attack on the ships of the Prodence and Dressrosa Kingdoms compared to his own. After the Seven Warlords of the Sea system was disbanded, Helmeppo joined Koby and other Marines in Amazon Lily to arrest Boa Hancock. However, he was turned into stone by Hancock's Mero Mero no Mi power, only to be saved by Silvers Rayleigh's intervention. When Koby was later kidnapped by Marshall D. Teach, Helmeppo and Hibari sought out the help of Prince Grus, a fellow SWORD member and Marine HQ Rear Admiral, to rescue him. Garp then arrived and urged Helmeppo to join him on a mission to Hachinosu to rescue Koby. Helmeppo expressed his frustration at being powerless to stop Koby's abduction, but ultimately agreed to join the mission.