Guy is appointed as the head of Slytherin house, after the sheriff of Nottingham somehow becomes the new headmaster of Hogwarts. He immediately makes a new rule saying that all pureblood students must transfer to Slytherin if they want to stay in school.
Guy abuses his power to get Marian to have detention with him. He asks her to help him overthrow the sheriff and have lots of little Gisbornes with him, but she says she needs to think about that. She refuses to pose for a nude portrait, so Guy gets Meg to wear a mask that looks like her. Robin Hood, Harry Potter and the other protagonists come barging into the room and think he's painting Marian. After a skirmish where Meg is accidentally impaled with a sword, Guy escapes by jumping out a window.
When the heroes return with Dumbledore to steal the Mittens of Power, Guy gives Allan detention and convinces him to become his personal evil minion. After the sheriff and Umbridge are kidnapped, Voldemort crucioes Guy into unconsciousness and Allan has to put him to bed.
Guy is standing guard outside the castle when Dumbledore and Gandal arrive at the school, disguised as French street musicians. He tells them to GTFO, resulting in them casting curses on him and hurling him in through a window.
Guy and Allan attend the party Dumbledore throws in celebration of having captured the sheriff and Umbridge. They are temporarily improsoned themselves, but manage to convince Meg to release them and join their side. Guy tries to comfort Meg, who has had her heart broken by Edward Cullen. He first tries to hook her up with Allan, but when she complains about Allan being short, he claims her for herself. After polyjuicing her to look like Marian, he tells her to infiltrate the outlaw's camp and murder Robin.