Assassin's Creed: Revelations. In a recent Game Informer piece, four of the game's multiplayer characters -- the vanguard, the guardian, the bombardier, and the sentinel -- are introduced, each with specific ties to the game's main single-player story.
The guardian is a member of the Templars, calling himself the "Guardian of Truth," and plays an unknown role in the forthcoming game. The sentinel is more fleshed out, with Ubisoft revealing to GI that he is a former Assassins Guild member who defected after the Ottoman Empire and the Assassins reached a truce; he'll be seeking revenge at some point during the main campaign. The last two remaining characters -- the bombardier and the vanguard -- are similarly of Templar loyalty, lured in by money and power.
How all of these folks factor into Ezio's final adventure remains to be seen, but each will show up in both the main campaign as part of the story and in multiplayer as playable characters. If Brotherhood is any indication, though, we'll see another half dozen characters added to the mix sometime before launch this November.