Perhaps the most obvious examples of Jem's need for fairness concern the trial of Tom Robinson. After Bob Ewell inflamed the spectators with his declaration that he had
"... seen that black nigger yonder ruttin' on my Mayella...,"
Jem initially backs Reverend Sykes' suggestion that the testimony wasn't fitting for Scout's young ears, but he convinces the pastor to let her stay. After all,
... nothing could make him leave now...
and Jem knew that it was only fair to let Dill and Scout stay, too. After the trial, Jem is distraught because of the jury's verdict. "It ain't right," he repeats over and over. He believes that the jury has acted unfairly, and he believes that Atticus's friends in the town have deserted him. He appreciates that Judge Taylor has appointed Atticus to defend Tom, recognizing that Taylor wanted a fair trial; but when Atticus explains that a white man's word is always taken over a black man's in a court of law, Jem responds that it "Doesn't make it right."
Jem learns about fairness from Boo Radley as well. He desires to send Boo a thank you note for the items left in their secret hiding place, but Jem discovers just how unfair Nathan Radley can be when he cements the knothole in order to prevent any further communication between Boo and the children. Jem even decides it would be unfair to Boo to invade his privacy by returning the blanket that he placed upon Scout's shoulders, and he keeps his promise to Atticus--"I ain't gonna do anything to him"--by never again attempting to "make Boo come out."
Jem soothes Scout with a Tootsie Roll after Aunt Alexandra refuses to allow Walter Cunningham Jr. to come to the Finch house. Jem knows their aunt's decision isn't fair, and he wonders aloud why
"If there's just one kind of folks, why can't they get along with each other."
His sense of fair play extends to animals in Chapter 25 when he cautions Scout not to kill the roly-poly, recognizing that the helpless creatures "don't bother you."