Subject Delta is an Alpha series Big Daddy and is the main protagonist in BioShock 2.
He was one of many divers that were (involuntarily) a part of the Alpha Series project, which converted them into what are known as Big Daddies. He was the first successful subject to be bonded with a Little Sister. The Little Sister in question being Sofia Lamb's daughter, Eleanor.
During New Year's Eve, 1958, Eleanor and Delta were completing their ADAM harvesting round when they were attacked by Splicers. Delta was unable to protect Eleanor because one of the Splicers used Hypnotise on him. It turns out that Sofia Lamb has instigated the attack. She takes Eleanor and forces Delta to shoot himself after removing his helmet.
Delta is resurrected at a Vita chamber thanks to the help of a group of Little Sisters and proceeds to find and rescue his 'daughter' Eleanor, which means he comes into direct conflict with Sofia, who sends her forces to kill him, including the Big Sisters. Eventually she manages to trap Delta, but doesn't kill him so that he is unable to ressurrect at a Vita-Chamber, which allows Eleanor to eventually free him.
This causes Lamb to spiral into depression, realising that all of her plans have come to nothing because of the strength of Eleanor's bond to Delta. She decides to destroy Rapture, detonating explosive charges.
At the end of the game whether or not Sofia Lamb dies is determined by Delta's actions over the course of the game. If Delta rescues the Little Sisters and spares the three NPC moral choices, then Eleanor will be influenced to forgive her mother and save her from drowning. However, if Delta chose to kill some or all of the NPC characters and harvests the Little Sisters, then Sofia will drown at the hands of Eleanor before reaching the surface.
