Coby is a young Marine solider who was once a captive of the pirate Alvida. He was saved by Luffy, and now follows his dream to become an Admiral.
For a time, Coby was working for Lady Alvida who he came to know by accidentally boarding her ship. According to Alvida he was a great navigator. He on the other hand had always desired to join the Marines, but his fear of Alvida stopped that dream and nearly destroyed it. Then Luffy came and defeated Alvida. Luffy took Coby with him so the boy could follow his dream.
"If I succeed, that's fine. If I die, that's fine also"
-Monkey D Luffy to Coby-
In Coby and Helmeppo's title page mini-arc, the two become unwillingly involved in an escape attempt by Morgan, Helmeppo's father. Morgan attempts to escape and actually takes his own son as a hostage. Cody put his own life as risk when he was the Marines were ready to shoot at Morgan with cannons, despite he was holding Helmeppo.
Even though Morgan escaped. Their bravery and loyalty caught the eye of Vice-Admiral Garp. Coby and Helmeppo were taken to Marineford. Their duties were still only as chore boys, but through their dedication and practice. They were eventually taken in advanced training as proper marine soldiers.
Luffy and Coby would meet again on the island known as Water 7. The reunion was now a classic one of normal friends. Coby attempted to attack Luffy, but even with the new skills he had acquired. Luffy still won easily. In fact, Luffy had not even recognized his old friend until later. Coby had really grown up and changed since they last seen each other on Shell Town.
It was only now did Coby learn the connection between Garp and Luffy. Garp is the grandfather of Luffy and the father of Dragon, the Revolutionary.
It''s here that Coby tells Luffy of his new dream. To one day become an Admiral in the Marines.
Coby and Helmepopo report into the Marine HQ for the battle against Whitebeard's fleet . Three hours before the execution Coby hears Sengoku's speech about Ace's shocking past as the biological son of Gold Roger.
To make maters worse Luffy has arrived on Marineford, and Sengoku reveals that Luffy is Monkey D Dragon's son. Luffy charges for the gallowed that held Ace, but Coby tried to stop. This attempt easily failed. Luffy won with a single punch to the face that didn't seem to hold back in the slightest.
Near the end of the battle. Ace was killed at the hands of Admiral Akainu, and Whitebeard had also been killed by the former Shichibukai, Blackbeard. Still, the other Marines and Admirals pushed to kill as many pirates as they could. This in spite of all the Marines that lay on the battlefield dying without aid. Suddenly, Coby gathered the courage to scream out to end this fighting or more lives would be wasted. Akainu turned to attack Coby for defying the orders he has been given. The young man was only saved by the appearance of the Red-haired Pirate, Shanks. In the presence of this man. Coby quickly passed out. He had no idea of what would come to pass.
One Piece: Episode of Luffy - Adventure on Hand Island (OVA)
One Piece: Episode of Luffy - Adventure on Hand Island is an OVA special.