Big Brothers are engineered Humans created to be soldiers for Grandfather's army. They are made from orphaned children who are genetically conditioned and outfitted with armor and weaponry.
They are similar to a cross between Big Daddies and Big Sisters. Able to use brutal strength as well as speed and plasmids, Big Brothers are deadly in combat. They are equipped with strong armor grafted to their skin and an arsenal of different weapons including harpoon guns, rivet guns, Arc Cutters, and the infamous Rail Guns.
The Big Brothers act as deadly agents trying to restore balance to Rapture as Grandfather tries to rebuild. When a Big Daddy or Little Sister is under attack or killed, a Big Brother will appear to restore balance however needed.