What has influenced Holden Caulfield's character development and in what way(s)?

Holden Caulfield's character development is mostly influenced by his little brother Allie's death 2-3 years previous to the current time period of the story. Throughout Holden's adventures, everything always seems to come back to the fact that he hasn't dealt with the death of his brother in a healthy way. He never got the mental and emotional help he needed after Allie's death; and this is the foundation upon which his mental breakdown builds. For example, Holden does not take Allie's death well from the moment he died, as shown in the following passage:

"I slept in the garage the night he died, and I broke all the goddam windows with my fist, just for the hell of it. I even tried to break all the windows on the station wagon we had that summer, but my hand was already broken. . ." (39).

Clearly, Holden needed immediate psychological help, but he only received a cast and was shipped back to prep school where he would wallow in grief alone. After that, Holden can't seem to apply himself to his school work and he sees everyone as phonies--fake people he can't trust. When he gets kicked out of his third school, Pencey, this is where the book starts and there has already been too much time pass with Holden stewing in his grief. The effect? Holden goes AWOL (absent without leave) and places himself in some scary and life-threatening situations for three days as he avoids facing his parents about getting kicked out of school again. 

Phoebe, his little sister, is the only person with whom Holden discusses Allie in detail, as follows:

"I know he's dead! Don't you think I know that? I can still like him, though, can't I? Just because somebody's dead, you don't just stop liking them, for God's sake-especially if they were about a thousand times nicer than the people you know that're alive and all" (171).

The above passage shows Holden trying to come to grips with his feelings that no one could ever be as nice or as real as his brother Allie. Everyone else is a phony except for maybe the nuns that he met having breakfast at a diner one morning and his little sister. If everyone was not phony in Holden's mind, then he probably would have reached out to his parents or a school counselor for help; but as it was, he felt alone and needed to take matters into his own hands and run away.