John Giant

Vice-Admiral John Giant (ジョン・ジャイアント中将 Jon Jaianto Chūjō) is one of the Marine officers in the Marine Headquarters.

Post Arlong Arc
During the announcement of Luffy's first bounty. At that time he was giving a speech regarding the "Justice". He is currently the only seen senior officer of the headquarters and who is assigned to lead the dispatchment of the Buster Call fleet, when needed.

Enies Lobby Arc
He was seen again, shouting orders after the Buster Call was signaled. When the signal was received a report was given directly to him, implying his high status at the headquarters,as well as the fact that he has the powers to command others of the same rank (via the five Buster Call vice-admirals).

Trivia

 * His name and rank are revealed in One Piece Blue.