Big Daddies are genetically enhanced human beings who have had their skin and organs grafted into an enormous diving suit. Their primary purpose is to protect Little Sisters while they escort them around Rapture. If all of the Little Sisters have been either harvested or saved in a level, Big Daddies will wander to a vent like usual and bang on it. However, a Little Sister will not come out, causing the Big Daddy to groan in frustration and bang again before letting out a sad moan and walk away in an attempt to find another Little Sister. Big Daddies communicate by uttering haunting sounds similar to that of whale calls.
Through genetic splicing, the humans inside the heavy suits were given enhanced strength and agility. Their suit exoskeletons contained the life support machinery needed to keep them alive during their duties, to allow a Big Daddy to lift heavy weights and do other strenuous tasks, and to help resist the cold and pressure of the deep ocean. Big Daddies can be seen doing maintenance repairs both inside and outside in the ocean. Big Daddies were also programmed to periodically return to facilities like Hephaestus for routine maintenance check-ups.
Alpha Series were the first Big Daddies and did not go through the extensive grafting process of later models, though still heavily spliced. They are unique in that they can use Plasmids and are bond to a single Little Sister. Because of this, they are believed to not have undergone some of the modifications later Big Daddies have, such as the application of pheromones. Alpha Series communicate with low, inhuman grunts and roars. They enter a permanent state of wrath if exposed to prolonged separation from their Little Sisters.
Standard (Production Model) Big Daddies have a bioluminescent chemical substance which fills the helmet, displaying its state of emotion or mind.
Bouncers are clad in a very distinct heavy diving suit, the entire torso of which is covered by a thick metal carapace. The head protrudes directly forward from the upper chest area and is encased in a hemispherical helmet studded with eight lit portholes. A large, fully-functional conical drill is attached to the right arm just forward of the elbow and is used for powerful melee attacks. The Bouncers get their name from their will to protect their bonded Little Sisters.
Rosies are clad in a basic heavy flexible fabric diving suit, the upper torso of which is covered by a high-gauge metal carapace. The head is guarded by a thick metal diving helmet welded to the torso carapace and has three large lit portholes. They wield large Rivet Guns and Proximity Mines at long range, and use the Rivet Gun for powerful melee attacks as well.
Rumblers are the newest type of Big Daddy, designed by Gilbert Alexander, his arsenal includes deployable Mini-Turrets and a shoulder mounted RPG that shoots non-guided rockets. This Big Daddy type seems to be more mobile than the other two types, although less armored. Its combat style gives it an advantage in open spaces, unlike its two counterparts, and it seems to fare well in long to mid-range combat. Its attacks are designed to keep the player on the move, and it can quickly turn the environment to its advantage and give the player as little cover as possible. A very formidable foe to say the least.
Lancers are much slimmer, slightly taller, and more decorated than their counterparts. He holds an Ion Laser, and appears to be a finished version of the Deco-Rosie, as seen in concept art and as a statue in the main campaign. This is the only type of Big Daddy the player encounters in the Minerva's Den DLC aside from the Alpha Series.