Penn Epner is a character in, and true main antagonist of Season 4. He works as a pizza delivery guy and is a member of the "Murderheads".
Penn is a bit of a "big head" personality- thinking he is smarter and savvier than others. He is also a bit of a fame whore and loves to be the center of attention. He lives to be on T.V., or interviewed, especially in regard to the bombing.
Penn also gets easily butt hurt. He doesn't like it when people taunt or make fun of him, particularly Don McNotten. He likes to be right and is upset when he is not.
Penn is first seen in the episode, Spring Break Forever, when he is delivering a pizza to the Sea Sprite. He has a friendly rapport with owner, Sul Ross. Penn leaves the motel right as the first bomb goes off. He gets hit by a nail in his back.
Penn proceeds to go on T.V. talking about the horrific incident as both a witness and a victim. He seems to love the attention and talks to multiple networks.
Meanwhile, Penn is a member of a group of murder-mystery solvers, called the Murderheads. They meet regularly to discuss and research different cases. They decide to research the Neptune bombing. Penn even sends in the shrapnel nail for testing.
Throughout the season, Penn works as a pizza delivery man- delivering pizzas to owners on the boardwalk, Veronica and Keith, and everyone in Neptune. In general he seems well liked, although some find him a bit annoying.
Penn approaches Keith and Veronica several times about joining forces, to research the bombing case. At one point, Penn even bugs Veronica and Keith's office to hear information. Veronica finds Penn annoying and declines his requests until the end, when she agrees to let him help out with some research.
Penn and a fellow Murderhead seem to find a clue in the case. They kiss and start casually dating. Penn had a crush on her for a long time and was hoping this would happen.
Veronica and Keith, in their investigation, come to the conclusion that Penn is the likely bomber suspect (but only for bombs 3-5). Penn maintains his innocence, hires a lawyer, and even goes so far as to hire Veronica and Keith to prove his innocence.
Penn kills fellow Murderhead, Don McNotten, to try and frame him for the bombings. He writes a confession letter and everything. Everyone almost buys the story until they find out it was a set up and Penn is actually guilty of three (almost four) bombings. Penn gets arrested and goes to jail.
Before Penn's arrest, he had also planted a bomb in Veronica's car. The bomb went off and killed Logan.
There was later a documentary about Penn and the bombings, leading to much debate over whether he was the real bomber or not. Penn's dream of being T.V. famous became a reality.