Leper Lepellier - What we know about Leper is done through indirect characterization in A Separate Peace. Leper is peaceful and loves nature. Most students make fun of him. Ironically, he is the first to enlist in the army after seeing a video on the ski patrol. After receiving a section 8 and getting kicked out of the military, he shows up as a witness against Gene at his mock trial. The fact that the insane person is the only one who sees things clearly when Finny falls from the suicide tree symbolizes a world gone insane with war.
Brinker Hadley - Brinker is your stereotypical prep school jackass. Brinker is an efficient politician and a ruthless administrator of justice, the opposite of Finny. It is Brinker's late night trial of Gene that forces the narrative to its climax and prompts the immediate events of Finny's death. Brinker combines the responsibility of adulthood with the cynicism of adolescence, at first exhorting his schoolmates to enlist and then backing off. The confrontation with his father toward the end of the novel highlights his bitterness toward the war and what it's done.
Cliff Quackenbush - Gene, after becoming the assistant crew manager, gets in a fight with Cliff on the first day of crew practice. Quackenbush is unliked by most and uses any opportunity to treat those inferior to him with disdain.
Chet Douglass - Finny's friend and main rival to become class valedictorian.
Dr. Stanpole - The campus doctor who operates on Finny--Dr. Stanpole sympathizes with Devon's youth forced to go to war.
Mr. Patch-Withers - Withers runs the school during the lenient summer session.
Mr. Ludsbury - Mr. Ludsbury is headmaster over Gene and Finny's dorm. He expects unwavering obedience from the boys.