What does enlisting mean to Gene? & what does the army come to mean to Leper?

Gene wants a sense of purpose to his life, and feels that enlisting will give this to him; he thinks hard about it over the night.

Gene realizes that he doesn't owe anything to either Devon or to his parents anymore; the choice of whether or not to go to war depends on his own inclinations, and he wants to go through with it. But, then he goes back to his room, and finds Finny there; suddenly, what he resolved to do no longer matters, as he has a purpose to stay at Devon again.

The foremost among the few who are not affected by the war is Leper, the odd, peace-loving acquaintance of Gene. Gene, upon meeting him in the woods on a snowy day, mistakes Leper for a "scarecrow," and the metaphor is actually a rather fitting one for Leper. Leper is not a person of action, nor is he particularly vital or lively like the rest of the boys; Leper prefers to remain on the periphery of things, in nature, like the scarecrow does. The irony is that Leper will actually join later.