One minor character that helps to develop the theme is Stradlater. This character is a strong contrast to Holden in a number of ways. The differences between the two boys helps to demonstrate the nature of Holden's issues in the text and show that it is a raw sensitivity in Holden that both keeps him from conforming and which sets him off on his journey.
Stradlater is a tall, macho, athletic all-American type who represents much of what Holden cannot be and detests. (eNotes)
Stradlater is anything but sensitive. He is a more-or-less normal teenager who does not feel particularly deeply. Stradlater focuses on looks and not on character, as we see in his penchant for shaving twice during his pre-date routines and in his willingness to cheat on school work.
Holden is quite different on each count here. He is not interested in looks - as evidenced by his odd choice of winter hat - and he is not willing to deceive or promote false appearances. Holden's opinion about phonies is excessively clear in the text, of course, and we can see that this is one of the significant differences between Stradlater and Holden.
Where Holden strives for authenticity (and an authentic achievement of some personal truth), Stradlater is perfectly willing to be false; to be shallow and to cheat. Holden responds to Stradlater's lack of sincerity with sharp criticism after Stradlater's date with Jane.
When Holden's queries about Jane escalate into a fight, Stradlater gives him a bloody nose, symbolizing how conventional modes of behavior typically prevail over the eccentric mode Holden pursues.
The "eccentric mode" that characterizes Holden's pursuit of a personal truth is a significant element of one of the novel's themes. Holden is unable and unwilling to simply conform to a standard perspective. He must forge his own, yet doing so proves to be highly difficult in a culture that champions conformity.