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.
John Giant is a giant, and towers a regular soldier by several times.[1] He wears a red hat with the Marines’ symbol on it, and a variation of the standard Marine coat with yellow colorings, red borders, and medals adorned on the left side. Unlike other high-ranking Marines, he wears his Marine coat with his arms in the sleeves. Underneath that, he wears a orange shirt. Overall, he appears to be a well decorated marine.
He also wields a katana that fits his size, slung over his shoulder.

(Source: One Piece Wikia)