Jack (known as "Jack the Raging Bull") is the owner of the Heaven and Hell Gas Station. He harbors a deep hatred of law enforcement, local or federal. He offers no help during the investigation and says nothing about any of the deaths throughout the course of the game.
He will only speak to Agent York when bribed with money, which will get him to open up about potential side quests available in Greenvale. When given $1000 total, his "chat with Benjamin Franklin" is what opens up the side quest York's Car which allows York to get his car fixed by General Lysander.
Jack is currently married to his third wife, Gina. Jack often visits the Galaxy of Terror to watch Carol MacLaine perform. He claims that Carol is the hottest woman around, even though he is married to Gina. Thomas gives a side quest later on in the game to try and get Jack to stop harassing his sister.