Leper is an outsider by his choice and by virtue of his interests and personality and how they are perceived by the others. Gene and Finny are not actively antagonistic toward Leper; they simply have different sets of priorities than he does, which means their paths don't cross frequently. When they encounter each other, they are companionable and respectful, if somewhat mystified by each other's reactions to the events and conditions around them.
During the invention process of Blitzball, Leper was not an active participant at first, but he was dragged into the game.
Leper Lepellier was loping along outside my perimeter, not noticing the game, taggling along without reason, like a porpoise escorting a passing ship. "Leper!" I threw the ball past a few heads at him. Taken by surprise, Leper looked up in anguish, shrank away from the ball, and voiced his first thought, a typical one. "I don't want it!"
When Gene is on his way to help dig the train tracks out of the snow, he encounters Leper, touring around campus on skiis and searching for a beaver dam. Gene is as surprised and baffled at what Leper is doing as Leper is uninterested in working and getting paid for the shoveling. They carry on, each in his own world.
With Leper it was always a fight, a hard fight to win when you were seventeen years old and lived in a keyed-up, competing school, to avoid making fun of him. But as I had gotten to know him better this fight had been easier to win.
When Leper, who is older than Gene and Finny, surprises everyone by enlisting so he can become part of the ski troops, his departure is greeted with bafflement at first, then with quiet nervousness as the boys consider how they will handle the approaching date when they become directly involved in the war. The apprehension is magnified when Gene visits Leper and comes to understand what had truly happened to his friend's mind.
Fear seized my stomach like a cramp...For if Leper was psycho it was the army which had done it to him, and I and all of us were on the brink of the army.
Leper's awareness of what had actually happened at the tree during the previous summer was a threat to Gene, but - true to the general attitude of discounting anything Leper said or did - his testimony was not completed and his hints at the truth were not understood for what they meant.
Leper was a casual acquaintance who happened to be in the right place at the right time, and so became important in some isolated events.