What event in Shift shows what kind of person Chris really is?
After Chris makes it to Washington state on his own without Win, thanks to Win's inexplicable disappearance in Montana, he returns home and starts his first year of college. He assume, of course, that Win is at college too and that the bizarre rift in their friendship that made him abandon Chris with a flat tire is still at work and so is not surprised that he doesn't hear from win. Chris is angry, indignant, shocked, disappointed, every negative emotion he could possibly be at what he views plainly as Win's massive betrayal of friendship.
These feelings all change and go through some nerve wracking upendings when the FBI shows up on the trail of Win's whereabouts. It is when Chris's thoughts and feelings shift from an inner focus (how he feels, what he thinks, his betrayed friendship) to an outer focus and concern for Win's life, safety and whereabouts that Chris shows what kind of person he really is. This is confirmed after he tracks Win down and, upon returning, tells FBI Agent Ward that he sort of found Win and definitely found himself.
[Below are reliable links to an eNotes synopsis of Shift, to Jennifer Bradbury's Web site and to an interview with Jennifer Bradbury.]