Big Bear is a member of the Grove Street Families and is one of the longest active members of the gang. Between 1987 and 1992, however, he developed a drug addiction.
Events of GTA San Andreas
Eventually, and by the time of the GTA San Andreas storyline, Bear had become the man servant of B Dup and does various chores, including cleaning the toilet. Bear had lost much respect and had turned on the Grove Street Families in order to get more drugs. However, Bear eventually confronts B Dup and decides to stop taking drugs. Sweet Johnson helps Bear with taking him to drug rehab.
In the game's internal files, in addition to the regular Big Bear model seen in the game, there is another unused model of him. This other model depicts a much fatter Big Bear, with slightly differently shaped facial features, but he is in the same clothes. It is unknown why this model is in the games files as it is never used in the game. It was probably going to be used in the beta Looking for Big Smoke mission, the regular Big Bear model is called "Bbthin" in the game files, which suggests that Big Bear would have been both fat and thin during the storyline.
In a deleted cutscene from the mission Beat Down on B Dup, Carl Johnson would take Bear to rehab with him explaining how he got into drugs. This would set up a mission where Carl would search for Big Smoke and would need to find an informant. Bear tells Carl and Sweet about a Russian Mafia gang member who would inform Carl of Smoke's whereabouts, which would set up End of the Line.
Both of these facts state that Big Bear originally had a bigger role in the game.