Koby

Koby was press-ganged into serving a pirate captain he was terrified of, and got out because Luffy turned up in the same stretch of ocean and could not understand why anyone would stay. His response was to join the Marines, the organisation that will eventually be sent to arrest the man who freed him. He said as much at the time, and neither of them minded.

What makes him work is the training. Koby is not gifted. He is handed to Garp, whose instruction consists largely of being hit, and he keeps turning up for it. By the time the story reaches the war at Marineford he can hear the thoughts of everyone on the battlefield, and he has the nerve to walk into the middle of one and tell admirals to stop.

Oda uses him as a measuring stick. Every time Koby reappears he is stronger, ranked higher and further from the boy who peeled potatoes, which lets the series show how much time has passed without stating it. He is also the rare Marine the story treats as sincerely decent rather than naive, which makes the eventual collision with his old friend genuinely awkward.