This is a tough question because Phillip changes so much over the course of the novel. I'm also not sure if the characteristic in question has to apply to Phillip over the course of the entire novel, or if it can be any characteristic with supporting evidence. I'm going to go with the latter.
In the beginning of the novel, Phillip is a complete jerk. He's rude, antagonistic to just about everybody, a racist, and believes that he is superior to most people. For example, Henrik is one of Phillip's friends, but Phillip doesn't exactly talk about Henrik in positive terms. Phillip flat out tells the reader that he finds Henrik annoying.
. . . Henrik had an irritating way of sounding official . . .
Phillip follows that thought up by describing what Henrik looks like.
His face was round and chubby. His hair was straw-colored and his cheeks were always red.
That's not exactly a glowing description of a friend.
Phillip doesn't hold back from his mother either. He's equally rude and insensitive to her.
Then I became angry and accused her of being a coward. She told me to go off to school. I said I hated her.
The above thoughts are all from people that Phillip at least supposedly respects. Here's what Phillip has to say about Timothy.
"You ugly black man! I won't do it! You're stupid, you can't even spell."
From the early parts of the novel, if I had to pick a single way to describe Phillip's personality, I would say that he is an insensitive jerk.