Coby

Coby is a Marine captain who trained together with Helmeppo under Vice Admiral Garp's supervision. He was formerly an involuntary cabin boy for the Alvida Pirates until he was liberated by Luffy, and he was one of the first people Luffy befriended on his travels. In his first appearance, Coby lacked any fighting skills. According to Alvida, the only reason she kept him around was because he happened to be good at navigation. During his mini-arc he is seen training alongside Helmeppo and Vice Admiral Monkey D. Garp. He has some knowledge of swordsmanship, as he was seen practicing with Helmeppo and Bogard. He has shown great growth as he started out with no fighting abilities to being able to handle the dangers of the Grand Line, particularly the New World. After two years, he reached the rank of Captain; it was revealed he has become more powerful as shown in Episode of Luffy when he shattered a gigantic metal disk with a single punch and performed Rokushiki techniques with ease. As a Marine captain, he has command over lower-ranking Marines and is in charge of his own ship.  Coby, along with Helmeppo, reported to Marine Headquarters as part of the fight against the Whitebeard Pirates, and is present for Sengoku's speech about Ace's past. After hearing that Ace is Gol D. Roger's son, the two men come to a conclusion that Ace and Luffy aren't biological brothers.

Coby

Frightened by the display of power from both sides and the ensuing bloodshed as the Marines and Whitebeard's forces clash, Coby and Helmeppo run away. By chance, they cross paths with Akainu and, from their hiding place behind a wall, witness the admiral brutally punishing another Marine for deserting. They overhear Sengoku's communication with Akainu through the admiral's Den Den Mushi, which reveals the fleet admiral's plan to ignore the schedule and execute Ace right away. It is then that they notice Luffy's ship falling from the sky. When Luffy attempts to charge the execution stand to rescue Ace, Coby blocks his path, determined to defend the Marines' objective even if it means the execution of his friend's brother. He tells himself that, if he cowers away from the situation, he will not be able to grow up. His resolve, however, is not enough: the Chief Petty Officer is no match for the captain of the Straw Hat Pirates and is defeated with a single punch. Luffy moves on, leaving Coby unconscious in the middle of the battlefield. Eventually, Coby wakes up, distressed that the war is still raging despite the fact that Portgas D. Ace has been executed. The trauma of the battle triggers his own Haki which comes on him in full force, allowing him to involuntarily sense every human presence nearby on the battlefield (described by Coby as voices "inside my head"). He is reduced to tears as he listens to the voices of the combatants disappearing one by one as they are killed. As the violence rises to a fever pitch, Coby screams at the top of his lungs, begging both sides to stop throwing lives away for a war that no longer has any purpose. The young Marine's cry causes a moment's pause in the battle and inadvertently saves Luffy's life. Kizaru, who was seconds away from destroying Trafalgar Law's submarine, stops to look at Coby, which gives the Heart Pirates the time needed to submerge and flee from the battlefield with Luffy on board. Akainu, unimpressed, prepares to kill Coby for wasting precious seconds of the battle. The young Marine fearfully steels himself for death, reassuring himself that he said what he had to say and that he has no regrets.

Marine Captain Coby

His life is saved by the sudden arrival of Shanks, who commends the young Marine for his actions, explaining that his few seconds of courage have drastically changed the future of the world. Coby immediately falls unconscious after his timely rescue from an otherwise certain demise. Coby is seen at Marine Headquarters healing center, surrounded by other injured soldiers and a healthy Helmeppo at his bedside. He is still clearly traumatized by the events, saying that he's "felt human presences too strongly" and that he "cannot calm down". Doctor Fishbonen tells him that what he is feeling is Haki. Doctor Fishbonen informs Coby that all Marines ranked vice admiral and above have Haki control, and that Garp should be able to help him. Helmeppo is envious that Coby seems to have advanced in skill, though Coby claims to not have known about the ability in the first place. The particular type of Haki Coby has gained is Kenbunshoku Haki.