Phillip first appeared in the The Bleedin' in Sweden, asking Frank if Bill could come out and play causing Frank to tell him to "get the fuck out of here!".
He was planning on going trick-or-treating with Bill dressed as Nickel and Pickle. The two were too scared to go however when Jimmy and his cronies threatened to beat them up if they did. Later in the night, Bill and Phillip decide to go out anyways, but Phillip creeps quietly back into his house as he sees the bullies instantly assault Bill just like they said they would.
Phillip is very similar to Bill in that they are both wimpy and a bit on the geeky side. Phillip has a constant whine to his voice, and does not seem to be as smart as Bill. Phillip also seems to be a hanger on to Bill. Phillip usually calls Bill "Billy".
It's revealed in "Bill Murphy's Day Off" that Phillip has asthma. He also admits he touched the soft spot on his younger brother's head when he was a baby, thinking it is why he cannot read in F is for Halloween. It is revealed in Season 2 that his father is Babe.
It's shown that Phillip most likely has severe and subtle psychological issues, most likely to him being unable to vent his anger and frustrations properly. Philllip reveals his collection of drawings to Bill that depict dismemberments and graphic deaths of those who anger him, he refers to this as his "kill book". A large amount of those drawings are dedicated to Jimmy.
"I feel dirty! (Runs away cackling)"