User blog comment:Willbachbakal/My BioShock Sequel Idea Dump/@comment-175.139.62.248-20130829142800/@comment-1330314-20130830215748

That's a great question!

In the singleplayer campaign the protagonist is Jim, an ex-lieutenant in Ryan's personal army who, after being nearly killed following the assault on Fontaine Futuristics, wakes up twenty-two years later. The main antagonist is Cassius Blaine, Jim's old boss and the man who ordered his death, who takes control of New Rapture, a hidden city on the coast built from the wreckage of the old underwater Rapture.

In the co-op campaign, the protagonists are Eleanor and Allison, two Big Sisters. Eleanor is Sofia Lamb's daughter, and Allison the child of two scientists who worked on the Little Sister program. I don't want to spoil too much yet about the antagonist(s), but they do change depending on the players' decisions.

In the multiplayer campaign, the protagonist is an unnamed Alpha Series Big Daddy who lost his Little Sister. The antagonist is the man who killed her, and who is also your mentor during the campaign. However, he warms up the player over time, and so loses his antagonistic qualities. The player can still choose to kill or spare him, though.