One who protects his Brother from the world, and other people.
The Big Brother or the family.
The one who people question when his brother(s) mess up.
Guy: Where is that little bro of yours.
Big Bro: Why
Guy: He needs to explain why he was short on his money.
Big Bro: Ill answer for him.
Guy: Who he hell are you?
Big Bro: My Brothers Keeper.