Subject 16 (given name Clay Kaczmarek) is one of the most mysterious characters in the franchise. He's appeared in several of the games, as a creation of the Animus who went rogue and keeps showing up the give the player information in the form of puzzles and hidden signs.
Not much is known about him, except for the fact that the original Clay went crazy from overuse of the Animus, but it appears that's about to change. The upcoming DLC for Assassin's Creed Revelations, entitled The Lost Archive, focuses on him, and will give the player some of the back story we've been missing. The trailer is available over on Eurogamer, but Youtube links keep getting taken down by Ubisoft. The DLC also will include 3 new character skins, a new mission, two new armors and a robe. Current rumor has it coming out on February 28th for 800 Microsoft points, but that's not yet officially confirmed.