Dennis Kobin is a friend of John and Lorraine. He is usually paired with Norton Kelly, and the two are described as "demented", and are always causing trouble. Of the pair, Dennis is the follower, and is nowhere near as malicious and conniving as the more assertive Norton. John says that Dennis is "not very bright", and notes that "he talks so slowly some people think he has brain damage". Dennis does hold the record for the being able to engage a stranger in the longest conversation ever in a "phone marathon" game that the four teens have created. The object of the game is to call a number at random and see how long one can keep the person who answers on the line. Dennis once called a lonely old woman and made up a story about needing advice about how to treat a skin disease which he had as a result of having had his nose bitten off by a rat when he was a baby. He was able to keep the woman on the phone for well over two hours.
Dennis's father works long hours and drinks a lot. When his mother is not home, the group sometimes gathers at his house to play the phone marathon game (Chapter 3). When John and Lorraine decide to throw a party at the Pigman's house, they invite Dennis, but caution him not to tell Norton because they know that Norton will take the opportunity to steal from the Pigman, and otherwise cause trouble. Dennis, however, can be counted on to be discreet and to follow instructions. Because John has asked him to, Dennis steals some alcohol to bring to the party from his father's whiskey cabinet, and supplies some soda mixers and glasses from his house as well (Chapter 13).