Jem's full name is Jeremy Atticus Finch. His middle name is from his father, Atticus Finch.
Jem is Scout Finch's brother. He is four years older than Scout. When the book begins, Jem is ten years old. The book ends when he is almost thirteen. He loves football. In the fight with Bob Ewell, his arm is broken. Jem's only concern is if he can still play football. To his satisfaction, he is.
As the book progresses, he matures and puts his childhood behind him, losing innocence. Jem crosses the line of childhood loyalty into adult behavior when Dill is found hiding under Scout's bed. To Scout's horrified disgust, Jem informs his father of the presence of the runaway Dill. He is sickened by the bigotry and injustice he witnesses. When he heard jury's guilty verdict of Tom Robinson for the rape of Mayella Ewell based on Tom's skin color, he vomited.
Jem and Scout were saved by their neighbor, friend, and gaurdian Boo Radley from the attack by Bob Ewell.
‘His face was streaked with angry tears.’-Chapter 22, after the trial.
‘...I watched the spark of fresh adventure leave his eyes.’-Chapter 8, after Boo Radley put a blanket on Scout.