Okay, so he's never actually called the pit monster, but he lives in a pit and we have to distinguish him from the tree monster somehow. All we ever see him do is pull Conor's mom into the pit with him, but he's pretty terrifying. He's the one Conor's seen in his nightmare.
Conor's former best friend told his classmates about his mom's cancer, and severed their BBF bond in one fell swoop. Like almost everybody else, she means well, but doesn't quite get what Conor's going through.
Conor's worst bully also just so happens to be a lifelong teachers' pet. He tells his friends he's the only one who can push and hit Conor, which is why the monster attacks only him in the cafeteria.
Harry's friend and Conor's bully, he's the one who makes a joke about Conor's mom being bald. Tacky much?
Meet Harry's other bully friend. We don't ever really learn anything about him, but he's always tagging along with Harry.
The Head of Class for Conor's grade, she gives him special treatment because his mom is sick, which he hates. She may be well-meaning, but that only goes so far.
She's Conor's English teacher who assigns the life writing piece.