Leper is 18 in the book, but from the time period that Gene told it Leper would most likely be 33
Leper's last name in "A Separate Peace" is Lepellier.
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Lepellier
I believe he is 17, or somewhere around that age.
watching ski troops
Leper Lepellier
Leper Lepellier is the first of Gene's classmates to enlist in the military during World War II. He chooses to join a ski patrol unit after being inspired by a presentation at Devon school.
Leper Lepellier
In the book A Separate Peace, Leper Lepellier suffers a nervous breakdown from which he does not fully recover from after enlisting in boot camp.
Gene's long-distance call in "A Separate Peace" was from Leper Lepellier, one of his classmates at Devon School. Leper calls Gene to inform him that he has enlisted in the military.
When Gene caused Finny to fall from the tree in the beginning, Leper saw what happened. Then when he enlists in the army he is overwhelmed. The burden of what he saw and the stress of the war are too much for him to bear and he goes mad. He later returns to Devon and tells the boys what he saw, leading to Finny's fall down the marble steps.
Yes, in the book "A Separate Peace" by John Knowles, Leper Lepellier does have a snail collection, which he keeps in a small woodenhouse he built to house them. This collection becomes a source of fascination and obsession for him during the story.