Brian is the youngest known son of Beverly Johnson and the younger brother of Carl, Sean and Kendl. It is implied that he bore a strong resemblance to Carl, as Emmet initially mistook Carl for Brian when they first met. According to Carl's flashbacks, it appears the two were close, and enjoyed creating mischief together.
Brian was associated with the Grove Street Families due to being the younger brother of the gang's leader, Sweet, but his role within the gang was unknown. Brian lived in the Johnson House in Ganton, Los Santos, until his death.
Brian died in 1987. The cause of his death was never revealed, though it is implied that he was killed by the rival gang and Carl was present but he did not attempt to help Brian.
Sweet blamed Carl for Brian's death, which resulted in Carl moving to Liberty City. Carl did not attend Brian's funeral, which Sweet confirms by saying: "That's another funeral you ran away from, fool. Just like Brian's."
Events of GTA San Andreas
Brian's death plays a major role in Carl and Sweet's relationship.
When Carl and Sweet are waiting outside Big Smoke's Crack Palace just before they attack it, Sweet attempts to follow Carl, but Carl tell hims that he has to go in alone and that he is doing this for Brian, as he "let him die".