In The Catcher in Rye, Holden is slowly losing his grip on reality. There is a big game going on at Pencey, but Holden isn't at the game. He is in his room reading a book when he hears someone coming out of the shower. He knows it is Ackley, and begins to tell us why he doesn't like him. 

Even without looking up, I knew right away who it was. It was Robert Ackley, this guy that roomed right next to me... He was probably the only guy in the whole dorm, besides me, that wasn't down at the game. He hardly ever went anywhere, He was a very peculiar guy... He was one of those very, very tall, round-shouldered guys-he was about six four- with lousy teeth. I wasn't too crazy about him, to tell you the truth.

This quote is quite interesting, because at the beginning of chapter 3, Holden tells us that he is a great liar, so to end this with "to tell you the truth" is pretty funny. Ackley probably reminded Holden of all the things he didn't like about himself. Ackley didn't go out much, like Holden. Ackley had a way of coming into Holden's room and touching his things, which Holden hated. The whole thing was that Holden was losing his reality and Ackley was a reminder of that.

There are certain aspects of Holden's personality that are very OCD or ADD, although these ailments would not be addressed until much later in time, somewhere around the 1960's. Holden mentions that not only is Ackley a slob, but so is Stradlater. Lots of teenagers are slobs and this is very common for a teenager to be a slob, however, Holden hates this quality in Ackley. Ackley does not seem to brush his teeth, pick up after himself and clips his toenails on the table. In the end, Holden is sickened by his behavior and this causes severe disklike, however, Holden deep down feels sorry for Ackley because Ackely is much like Holden himself, and cannot fit into society.

Robert Ackley gets on Holden's nerves, much like just about everyone else Holden encounters.  Ackley has awful teeth, which he never brushes, lots of pimples, and a lousy personality, according to Holden.  Holden also hates the fact that Ackley  comes into his room and picks up things that belong to him and his roommate, Ward Stradlater.

Check the link below for more information on this and the other characters in this book.  Good luck!