Symbolism, Imagery, Allegory
Monsignor Darcy gives Amory a very important lesson when he teaches him the difference between personalities and personages. According to Darcy,
"A personality is what you thought you were, what this Kerry and Sloane you tell me of evidently are. Personality is a physical matter almost entirely; it lowers the people it acts on." (1.3.171)
In other words, your personality is the superficial part of you. It's the part the other people see when they first meet you. It's your appearance and the things you say. For Darcy, your personality is not what matters in life. It's the spirit behind the personality that matters, which brings us to his concept of the personage.
Monsignor Darcy continues,
"Now a personage, on the other hand, gathers. He is never thought of apart from what he's done. He's a bar on which a thousand things have been hung—glittering things sometimes, as ours are, but he uses those things with a cold mentality back of them." (1.3.171)
In other words, personality is what you say and how you appear; personage is what you do and what you stand for. For Darcy, a personage is way more important because it gets to the core of whether a person is good or bad.
Unfortunately, Amory Blaine spends an awful lot of this book worrying about his personality when he should be worried about becoming a better personage. But Amory's final lines—"I know myself, but that is all"—suggest that he's coming around to the whole "personage" side of the human equation.
After all, This Side of Paradise is very much a coming-of-age story, and what is growing up if not abandoning the idea of yourself (and how you want to appear to others) in favor of embracing the fact of yourself (and what you want to do)? Yeah, Shmoopers, we went there. We just got deep.