Every character in Tim O'Brien's "The Things They Carried" changes as a result of war. The basic central theme of the story is precisely how men and women change after being touched by the unique and sad experiences that arise as a result of war.
Rat Kiley is perhaps the person who is most affected, although O'Brien is careful in not making one man's experiences seem of lesser or more importance than the others. All men are affected by war in one way or another.
However, what makes Rat's experience unique is that he is the life of the crowd. As a medic, he carries the cool things such as medicines, sweets, and even comic books. His humor tends to make things easier in a difficult situation. However, when Ted Lavender is shot, we begin to see the change in Rat in the way that he internalizes what is going on.
... right then Ted Lavender was shot in the head on his way back from peeing. He lay with his mouth open. The teeth were broken. There was a swollen black bruise under his left eye. The cheekbone was gone. Oh.. Rat Kiley said, the guy's dead. The guy's dead, he kept saying, which seemed profound -the guy's dead. I mean really.
It is after the death of his friend Curt Lemon that Rat completely changes.He shoots a baby buffalo in the field, and even tries to shoot his own foot to get sent back home from the battlefield. Rat's is perhaps one of the saddest changes in the story because it shows how deeply war can break or make a man.