Who is Billy from Trainspotting and what is their importance?

Billy is Mark Renton's brother, a soldier in the British reserves and a loyal Unionist. He and Mark rarely see each other, and he has a wife Sharon who is pregnant with his child in the novel. Billy is a member of a group of semi-professional gamblers who pool their money to play in the gaming clubs. One day, a member dies suddenly while holding the communal pot. Billy knows another member of the group is sleeping with the dead man's wife and proceeds to beat the truth of the man's death of him.