Ben Tomlin is the 13-year-old protagonist and narrator of Half Brother. He has curly, sandy blond hair, resembling the 1970s British actor Michael York. Although his peers find him attractive, Ben does not see himself this way.

In many ways, Half Brother is a coming-of-age story, following the growth of both Ben and Zan. At the beginning of the story, Ben has significant anger issues, often responding to frustration with violence or by shutting down. As the story progresses, Ben learns to control his temper better, although he has minor setbacks.

Starting at a new school, Ben tries to establish his social status. He aims to be the "dominant male" and succeeds for much of the book. However, after his relationship with Jennifer ends, Ben self-isolates and goes "from dominant male to low-ranking insect" only to return to dominant male status after Zan is sent away (231).

For much of the story, Ben tries to control the environment around him in contrast to the lack of control he has at home. He keeps a project log to track Jennifer's habits, hoping to improve his chances with her. He refuses to follow teachers' instructions and argues with his parents. As the novel progresses, Ben realizes that "you couldn't really control anyone or anything" (307).