Caleb is Tris's brother, a.k.a. her only surviving family member, a.k.a. the only family member who didn't selflessly die to save Tris. She probably wishes he would just die, though, because he ends up siding with Jeanine over her. He passes private information to Jeanine about Tris in order to further her research into the Divergent, and Tris pretty much writes him off as an evil traitor forever. We can totally picture her dramatically hissing, "I have no brother!"
At the end of the book, Caleb's fate remains up in the air. Will he be executed as a traitor? Will Tris even care?