Subject Sigma

Subject Sigma (Symbol: Σ) is the protagonist of the Minerva's Den DLC BioShock 2. He has been revived by Charles Milton Porter and Brigid Tenenbaum, who sent out a distress call and who works with Sigma during the course of his adventure.

It is revealed that Sigma is Charles Milton Porter, who was previously jailed in Persephone due to Reed Wahl creating a fake audio recording of him so he would be accused of treason. Porter was then turned into an Alpha Series Big Daddy with no recollection of his previous life. His creation, The Thinker, found a way to revive Porter and helped him escape from Rapture by assuming Porter's personality as a familiar face to guide him.

Source was the BioShock wikia page.

Battle vs. Subject Sigma (by Lasmoore)
The battle starts out with Sigma and Dr. Tenenbaum getting to dry land, carrying out examples of the technology with them. Duke spots Tenenbaum and Sigma walking up from the beach and mistakes Sigma for an alien taking Tenenbaum to an alien ship. Duke shoots Sigma with his Alien Blaster and he falls down Tenenbaum runs away to a safe place. Duke pulls out his M1911 out on Sigma and says "Hail to the king, baby!" and shoots Sigmas helmet, the bullet deflects and hits Duke in the shoulder. Sigma gets up and swings his Drill at Duke, knocking him on the ground. Sigma then runs away to find Tenenbaum. Duke gets up and says "No you dont you c0%k sucker!." Duke shoots two missiles at Sigma with his devastator. Sigma hears the missiles coming and turns around and catches them with telekinesis. Duke says "What the unholy f$$k!" and Sigma hurls them towards Duke but he ducks and they miss. Duke says "Take the bitch motherfucker, i dont care anymore." Sigma, angry atwhat Duke said, Throws a Gravity Well in between them. Duke then gets sucked into a mini sand storm and then gets hurled towards Sigma. Duke says "You want some more, huh!" Duke kicks Sigma on the ground and thinks he could kill Sigma with a punch and punches Sigma's armor, only leaving a dent. Sigma shoots Duke with his rivet gun. He then pulls out his Ion Laser and swtches to Burst Cells and Swings Duke up in the air with his Drill and then shoots Duke into pieces with his Ion Laser. It then starts raining and Sigma walks away.

Expert's Opinion
Due to Subject Sigma's superior arsenal, larger size, and tougher armor, he triumphed over Duke Nukem.