The Big Brother is an enemy in the Bioshock series. The Big Brother will appear after you defeat a Big Sister.
The Big Brother is a grown up Little Brother whom survived the events of the civil war, Bioshock and Bioshock 2. The Big Brother suit was designed by at the start of Bioshock 2. The Big Brother will appear after you defeat the Big Sister (If the Big Sister can't finish the job then he'll finish it for her). Althought the Big Brother does'nt serve the Rapture Family he will only fight for Family cause's (Mental Status: Thinks the Big Sisters are family). There is more than one Big Brother