Victor "Sully" Sullivan is the longtime partner of Nathan Drake, and a treasure hunter as well. Sullivan has mentored Drake for some time, and is a father figure for him. Richard McGonagle voices Sullivan for all three games in the series but appears only briefly in the second. Sullivan first appears near the beginning of Uncharted: Drake's Fortune when he rescues Drake and Elena from a pirate attack at sea with his seaplane.
He appears briefly near the beginning of Uncharted 2: Among Thieves. He also appears as a playable character in the multiplayer mode of the game.
Sullivan is a business partner of Drake's. He was meant to act, not only as a partner, but as a mentor to the main character, and to provide insight into his criminal activities. In both games, Sullivan wears a bowling-style shirt, dockers pants, and chews on a cigar wherever he goes. He is constantly in enormous debt, but is likable enough to pull people into investing in his ventures.
Sullivan also appears in the PS Vita game, Uncharted: Golden Abyss
Source: Wikipedia, "List of Uncharted characters", available under the CC-BY-SA License.