When Amory gets to Princeton, Kerry Holiday is the first guy he meets. Which is lucky for Amory, since Kerry is a good ally and an all-around easy person to get to know. As the book tells us,
Kerry was tall, with humorous grey eyes, and a sudden, attractive smile; he became at once the mentor of the house, reaper of ears that grew too high, censor of conceit, vendor of rare, satirical humor. (1.2.79)
In other words, Kerry doesn't take life (or himself) too seriously, and this attitude has a profound influence on Amory, who takes both life and himself way too seriously. From day one, Kerry draws Amory more toward a healthy balance of ego and modesty, and helps him develop a sense of humor along the way.
Even though Kerry teaches Amory a lot, Kerry also has a level of confidence in himself that Amory seems forever doomed to be lacking. When Kerry pays only a fraction of his restaurant bill, for example, he says to the waiter, "No mistake!"and then,
shaking his head gravely, and, tearing [the bill] into four pieces, he handed the scraps to the waiter, who was so dumbfounded that he stood motionless and expressionless while they walked out. (1.2.340)
Dubious morality? Sure—dine 'n' dashing is a criminal act. But he does it with a suavity and self-assuredness that Amory admires. Amory is way too self-conscious to ever try strutting out of a restaurant, even if he had paid the bill.
For Amory, Kerry becomes a symbol of someone who doesn't worry too much about life's deeper questions and who simply enjoys the moment. Unfortunately, Kerry goes off and dies fighting in World War I, just like many young men of his time.