Subject Delta has the features of a Big Daddy, such as the vocal cord modulator surgery and large diving suit. Like all Alpha Series Big Daddies, he retains a large degree of free will, but he is forced into a protection bond with a single Little Sister. As a prototype, Delta is granted use of the Bouncer's drill (which is detachable and goes over the hand), in addition to other weapons, such as the Rosie's rivet gun, and a Hack Tool that allows him to remotely hack from a distance. He also has the ability to use Plasmids, and can wield them simultaneously with his weapons. Delta's suit was designed specifically to allow the efficient use of plasmids. His gloves have openings on his fingertips and palms to accommodate the necessary contact with the environment projectile plasmids need, his sleeves have IV ports for ADAM injection and a tube for transporting EVE from his tank straight to his wrists to allow swift replenishment even during combat. Being less bulky than the production models gives Delta greater speed and agility than other Big Daddies, at the sacrifice of some of the raw strength and endurance they possess. Also, First Aid Kits and EVE are fed intravenously to Delta, meaning he can continue fighting while replenishing his EVE reserves.
Throughout the game Delta must defeat other Big Daddies and take their Little Sisters, at which point he is given the option to either "harvest" or "adopt" them. Harvesting functions the same as it did in BioShock, whereas with the new "adopt" option, Delta puts the Little Sister on his shoulders, forming a temporary bond with the girl. He protects her as she extracts ADAM, and the Little Sister will give him some of the ADAM she harvests from Splicer corpses as a reward. After she has finished, Delta can send her to a Little Sister Vent, where he will have the final choice of saving or harvesting the Little Sister.
Before his arrival in Rapture, the man who would become Subject Delta was a deep sea diver who conducted an investigation of the many ship and submarine disappearances in the Atlantic Ocean southwest of Iceland. While surveying the area in an advanced diving chamber he accidentally discovered the hidden city. He was accepted into Rapture, becoming somewhat of a celebrity for his heroics and nicknamed "Johnny Topside" by articles in the Rapture Tribune.
However, Andrew Ryan did not trust his story and was convinced that he must be a government agent sent to spy on Rapture. Stanley Poole, a journalist who had written a story on "Johnny Topside" turned him in to Ryan, who immediately had him incarcerated at Persephone Penal Colony and erased all documentation of him from the city. Thus, he effectively disappeared from the public eye like all of the other citizens who had dared to speak out against Ryan.
As a prisoner in Persephone, he was contracted out by Augustus Sinclair as a test subject for Ryan Industries' production of plasmids and Big Daddies. He was labeled "Subject Delta," (Δ) and was transferred to Fontaine Futuristics to be used in various plasmid experiments, performing in the Plasmid Theater to amaze crowds of oblivious Rapture citizens. Eventually, he became one of the primary candidates for conversion into a Big Daddy. He endured strenuous genetic and physiological conditioning, losing much of his free will in the process, and became the first Alpha Series protector to successfully bond to a Little Sister. His bond partner was Eleanor Lamb, the daughter of Rapture's ethical psychiatrist Sofia Lamb who had been incarcerated in Persephone not long before.
At some point during New Year's Eve of 1958, Eleanor and Delta began their normal rounds of harvesting ADAM and passed through a New Year's party. Eleanor was attacked by four splicers, and as Delta fought to protect her, one of the splicers used the newly-developed Hypnotize on him. The splicers were Sofia Lamb's agents, and Lamb used the plasmid to force Delta to kill himself in front of Eleanor, via a point-blank shot to the head with a golden Luger P08 pistol.
As Eleanor grew up in her mother's care, she still remembered her "Father," Delta, and longed for a way to bring him back. Through her influence over the other Little Sisters, she managed to find Delta's body and extract a gene sample. She used this sample to calibrate the Vita-Chambers to his genetic frequency. She also contacted Brigid Tenenbaum, requesting her aid in undoing the psychological conditioning that had been imprinted in Delta's mind when he was a Big Daddy. Finally, ten years after his death, her plan succeeded.
Roughly ten years after his death Delta is revived in a Vita-Chamber in Adonis Luxury Resort. Having no recollection of the previous ten years, Delta is unaware of his surroundings and has no knowledge of the downfall of Rapture. He is greeted by a distorted radio transmission from Brigid Tenenbaum, and receives a strange vision of a girl's face with the word "Father" when he finds a gift of the Electro Bolt plasmid. He returns power to the broken generator in the level and Tenenbaum speaks to him more clearly, promising to tell him what she knows of his past if he will travel to meet her in the ticket booth of the station in the Atlantic Express train yard.
Delta begins his journey to her, but is assaulted by splicers loyal to The Rapture Family in a trap laid for him by Sofia Lamb. He survives, but Lamb continues to follow his progress through the city, contacting him through the radio and calling splicers to attack him. Eventually, he reaches Tenenbaum and she briefly tells him of the situation in the city and what little she knows of his own background. She escapes while Delta takes on a wave of attacking splicers and tells him to follow Augustus Sinclair's instructions to reach Fontaine Futuristics and find his bonded Little Sister, Eleanor.
Subject Delta meets up with Sinclair in the train station of Ryan Amusements and clears the ice blocking the train to continue on to Pauper's Drop. There he is forced to find Grace Holloway in The Sinclair Deluxe tenements to get the security key from her. As he makes his way toward her he learns more about her history with Eleanor and why Grace holds such a strong grudge against Delta. When they finally meet face to face, Delta has the choice of whether to kill Grace or show mercy by letting her live.
After he obtains the key from Grace, Delta and Sinclair continue in the Atlantic Express train on their way to Dionysus Park. However, as they approach the Park the train is hit by a torpedo launched by Simon Wales in Siren Alley. Delta is thrown from the train car and lands on the ocean floor, while Sinclair continues on to Dionysus in the damaged craft. Dionysus Park is flooded, and Delta must hurry to drain it from the pumping station in Siren Alley before the oxygen runs out in Sinclair's train car. However, the pumping station is heavily guarded by Simon Wales and the loyal Splicers of The Rapture Family. After Delta defeats them, he accesses the pumping station and drains all the water from Dionysus into Siren Alley. He, then, walks from the seafloor entrance of Siren Alley to Dionysus Park.
In Dionysus, Delta meets Stanley Poole, a spy for Ryan who was instrumental in Ryan's arrest of Sofia Lamb. Stanley is locked in the security booth of the train station and refuses to let the train pass until Delta has dealt with every single Little Sister in the level. After Delta dealt with each Little Sister, Eleanor Lamb tells him about the memories hidden in the harvested ADAM that Stanley had not wanted to be discovered. Delta learns that Stanley was the one who flooded the entire park, killing all the people inside, that he was the one who handed Eleanor over to Ryan's men to be made into a Little Sister. He also learns that Stanley gave Delta to Ryan to be put in prison. Stanley opens the way for the train to leave, but Sofia Lamb unlocks the train booth, giving Delta the choice of whether to take revenge on him or leave him.
From Dionysus, Atlantic Express moves to Fontaine Futuristics, their destination. Delta hopes to find Eleanor here, but she is actually in Persephone, which can only be accessed by getting a genetic key from Gilbert Alexander. Alexander has gone insane, but left messages from before his insanity for anyone who would come in the future to end his life. Following his instructions, Delta enters the hidden labs of Fontaine Futuristics and sees what Sofia Lamb's philosophies have caused Alexander to become. Delta takes the genetic sample for the key from Alexander and his pre-insanity recording urges Delta to end his life.
Delta finally finds Eleanor in the Persephone Detention facility. However, she is sealed in a decontamination chamber. In an effort to kill him once and for all, Sofia Lamb suffocates Eleanor to stop her heart just long enough to break Delta's bond with her and begin his own death. Sofia's plot succeeds, but, with Eleanor's help, Delta is freed and works with Eleanor to try to escape in a submersible owned by Augustus Sinclair. In response to this, Sofia orders her loyal followers to detonate various charges around the overhanging complex of Fontaine Futuristics and Persephone, dropping the entire structure into a trench below. Eleanor, using the powers she had gained from the ADAM injection treatments her mother forced on her, teleports herself to the rising escape submersible just as the charges detonate. She reaches out and attempts to teleport Delta as well, but the explosives detonate before he gets close enough to grab her hand. However, Delta is close enough to the submersible to grab onto a railing on the outside and climb onto it as it rises towards the surface.
Delta is able to survive long enough to reach the surface with Eleanor. However, Delta's wounds and shattered bond with Eleanor become too great a burden for him, and by the time he and Eleanor reach the surface, he is breathing his dying breaths. Depending on the players choices throughout the game, one of four outcomes are possible. In the end, Eleanor either preserves Delta's essence within herself by extracting his ADAM, or she allows him to die.