Montague was a Slytherin student at Hogwarts, a contemporary of Harry Potter. He was a key member of the Slytherin Quidditch team, serving as a Chaser and later as its Captain. A member of Dolores Umbridge's Inquisitorial Squad, Montague is most significant for an incident where he was trapped inside a broken Vanishing Cabinet by Fred Weasley and George Weasley. His subsequent account of the experience provided Draco Malfoy with the crucial knowledge needed to use the cabinet to smuggle Death Eaters into Hogwarts during the Second Wizarding War. His first name, Graham, is never mentioned in the original novels.
Montague's presence is noted primarily during his time on the Slytherin Quidditch team. In Harry Potter's third year, Montague played as a Chaser against Gryffindor and was known for his aggressive and dirty playing style; Harry recalled him once trying to gouge out one of his eyes during a past match. His role became more prominent in his fifth year, when he was made Captain of the Slytherin Quidditch team. He also became a member of Dolores Umbridge's Inquisitorial Squad, using his position to bully other students. After attempting to unfairly take house points from Fred Weasley and George Weasley, the twins retaliated by shoving him head-first into a malfunctioning Vanishing Cabinet on the first floor of the castle. Montague vanished completely and was missing for weeks. He eventually reappeared, disoriented and traumatized, wedged inside a toilet on the fourth floor. He later recounted his experience, explaining that he had been trapped in a state of limbo, able to hear sounds from both Hogwarts and, faintly, from Borgin and Burkes in Knockturn Alley, where the cabinet's twin was located. In a desperate act of survival, he managed to Apparate out, despite not having passed his Apparition test. This event, while a mishap for Montague, became a cornerstone of Draco Malfoy's plan in his sixth year. Having overheard Montague's story, Malfoy realised the potential of the Vanishing Cabinets and spent the school year in the Room of Requirement repairing the one at Hogwarts. His success allowed a party of Death Eaters to infiltrate the castle, leading directly to the Battle of the Astronomy Tower and the death of Albus Dumbledore.
Montague is described as having a “heavy-jawed face” and a “gorilla-like build.” His personality was aggressive, arrogant, and confrontational. He was a ruthless Quidditch player who did not hesitate to play dirty. As a member of the Inquisitorial Squad, he demonstrated a clear enjoyment of power and authority over other students, aligning himself with Dolores Umbridge's oppressive regime.
The surname Montague is most famously associated with William Shakespeare's play Romeo and Juliet. The Montagues are one of the two central feuding families in the story. This likely reflects the intense house rivalry, particularly the bitter feud between Slytherin and Gryffindor, which often mirrors the Montague-Capulet conflict in its intensity.