Bronn is a sellsword of unknown origin, and a close acquaintance of Tyrion Lannister during the War of the Five Kings.
Biography
Origins
He is a sellsword who has had a rough childhood and supposedly he killed someone before the age of twelve. Little else is known about his background.
Working for the Imp
In A Game of Thrones, Bronn met Tyrion Lannister when Catelyn Stark ordered all local men in the inn he was frequenting to apprehend Tyrion and take him to the Eyrie, so he could be tried for the attempted murder of Bran Stark (although he was innocent). Bronn accepted this command but became close to Tyrion as he was escorting him through the Vale, much to Catelyn's chagrin. He later saved Tyrion's life during a trial by combat at the Eyrie. After they leave the Eyrie he finds Shae for Tyrion and accompanies them both to King's Landing.
In A Clash of Kings, Bronn became Tyrion's main guard. He was later knighted for his significant role during the Battle of the Blackwater.
Own agendas
In A Storm of Swords, Tyrion is being tried for the murder of Joffrey Baratheon. Bronn does not want to fight Gregor Clegane for a trial by combat, so instead he takes an offer from Cersei Lannister to marry Lollys Stokeworth.
In A Feast for Crows, he names his wife's son (a child born of rape) "Tyrion" to honor his patron. However, this leads Cersei to believe they are in cahoots together. She conspires with some members of House Stokeworth to put Bronn to death, but in his duel he slays his enemy with dirty tactics and winds up being named Lord Protector of Stokeworth.
Personality
Although he is a close friend of Tyrion, the latter is aware that most of the things he does are for money because he is a sellsword. He has a dark sense of humor and an amoral view on life.