Jem is 10 years old at the start of the book, 5 years older than his younger sister Jean Louise "Scout". In the book, his age ranges from 10-13. He is 4 years older than Scout. Jem is also the son of lawyer Atticus Finch. One day, Jem learns that Atticus has taken a case defending a black man named Tom Robinson who has been accused of rape. He, Scout, and Dill Harris hear Atticus' speech and see the result of the trial, with Robinson pronounced guilty by the court, thus ensuring that he went to jail. He later dies.
Sometime later, Scout is dressed up as a ham for a Halloween pageant at her school, while Jem agrees to take her, as school is not far at all. They both are near victims of an intended murder by Bob Ewell, the man who accused Tom Robinson of raping his daughter, Mayella Ewell. Jem gets hurt trying to protect Scout. Then, Arthur "Boo" Radley rescues both of them from Ewell and brings them to Atticus.
He dies in the sequel, Go Set A Watchman, however due the questionable canonicity of this book and its more widely-accepted status as a first draft, this fact depends on whether the reader decides to treat the book as a sequel or not.