Big Daddies (originally named Protectors, and occasionally referred to as Mister Bubbles, or Mister B, by Little Sisters) are genetically enhanced human beings who have had their skin and organs grafted into an enormous diving suit, with Alpha Series being an apparent exception to the grafting process, though still heavily spliced. Their primary purpose is to protect Little Sisters while they escort them around Rapture.Aside from the Alpha Series, Big Daddies communicate by uttering haunting sounds similar to that of whale calls. Alpha Series communicate with low, inhuman grunts and roars. For easier interpretation (as stated in the Deco Devolution art book), production model Big Daddies have a bio luminescent chemical substance which filled the helmet, displaying its state of emotion or mind.
Little Sisters (originally named as Gatherers by the game developers) are young girls who have been genetically altered and mentally conditioned to reclaim ADAM from corpses around Rapture. Little Sisters are almost always accompanied by a Big Daddy. They generally are immune to damage, outside of being “harvested”, and have no offensive abilities. Attacking them, however, will incur the wrath of their Big Daddy protectors.
(I got the Info from the wiki directly because I didn’t want to type so much out)
