Tim Shepard is the leader of the Shepard gang, a group that is friendly with the greasers but even tougher than they are.
He isn’t one of Ponyboy’s close friends, but he’s part of the larger network of greaser-allied groups in Tulsa. His gang shares the greasers’ hostility toward the Socs and follows a similar code of fighting and loyalty, though they have a reputation for being more dangerous. Tim is also personally connected to Dallas (Dally) Winston, showing that Dally moves among some of the roughest people in their world.
Tim Shepard’s role is fairly minor in the novel, but he helps show how widespread and organized the greaser side is. The presence of his gang—along with others—makes the conflict with the Socs feel bigger than just Ponyboy’s immediate circle, turning it into a larger community struggle between groups.