" ...we forced him head-first into that Vanishing Cabinet on the first floor."
"But you'll get into terrible trouble!"
"Not until Montague reappears, and that could take weeks, I dunno where we sent him..."
-- Fred explaining to Hermione what he and George did to Montague (OP28)
"They've found Montague, sir, he's turned up jammed inside a toilet on the fourth floor."
"How did he get in there?"
"I don't know, sir, he's a bit confused."
-- Draco to Snape during Harry's 'remedial potions' session (OP28)
"Montague told me that when he was stuck in the Hogwarts one, he was trapped in limbo but sometimes he could hear what was going on at school, and
sometimes what was going on in the shop, as if the Cabinet was traveling between them, but he couldn't make anyone hear him ...
. . . I was the only one who realised what it meant - even Borgin didn't know - I was the one who realised there could be a way into Hogwarts through the Cabinets if I fixed the broken one."
-- Draco to Dumbledore on the Astronomy Tower (HBP27)
Slytherin, c. 1989 – 1996, Quidditch Chaser/captain
Member of Umbridge’s Inquisitorial Squad who made the mistake of trying to take House Points from the Weasley twins without witnesses, and consequently was stuffed by them into the first floor Vanishing Cabinet; reappeared some days later jammed into a toilet on the fourth floor, after which he was confused and disoriented for a long time (OP19, OP28, OP30).
- Montague extricated himself from the Vanishing Cabinet by Apparating out, even though he had never passed the Apparition Test (HBP27)
- Draco interrupted Harry's Occlumency lesson with Snape to tell him that Montague had been found trapped in a toilet pipe, and Umbridge needed help getting him out. Snape left Harry alone with the Pensieve containing his "worst memory" of being bullied by James and Sirius, and losing his friendship with Lily Evans, Harry's mother. Thus ended Occlumency lessons, and Snape was in a bad mood for weeks. (OP28, OP30)
- Hermione was the only one of the Gryffindors who worried about Montague. When she heard he was still "disoriented" after his time in the pipe and saw his parents arriving at Hogwarts to see him, she considered telling Madam Pomfrey what had happened "in case she can't cure him" (OP30). Ron and Harry were indifferent, and Harry was glad there was more trouble for Umbridge.
- Montague told Draco that while he was lost inside the Vanishing Cabinet he could hear voices at both the Hogwarts end and Borgin and Burkes in Knockturn Alley, where a second Vanishing Cabinet existed. Draco experimented on the Cabinet in the Room of Requirement and eventually figured out how to let Death Eaters into the castle through it for the Battle of the Tower (HBP25,HBP27).
Excellent Quidditch player and Slytherin team Captain. Also pretty good at Apparating himself out of tight spots.
Other canon notes and references
Foreshadowing: Montague's Vanishing Cabinet was wrecked by Peeves in Harry's second year. He didn't see it happen, but he heard Filch tell his cat, Mrs. Norris “That vanishing cabinet was extremely valuable! ...We’ll have Peeves out this time, my sweet —” (CS8)
Also in second year, Harry spent some time in Knockturn Alley counterpart to Montague's Vanishing Cabinet. He ducked in there to hide after accidentally flooing himself into Borgin and Burkes on his way to Diagon Alley (CS4). From inside the Cabinet, he watched Malfoy and his father Lucius enter the store and talk about raids on their home. That day Draco also looked at the Hand of Glory and the dangerous opal necklace, which he later used in plotting against Dumbledore (CS4, HBP12, HBP27, HBP28, HBP29)
We deduce that Montague is one or two years older than Draco Malfoy, since Malfoy wasn't made team captain by his fifth year. (Malfoy's parents bought him everything else...) -- MLW
Montague has no name in the books, but in the Harry Potter Quidditch World Cup video game, his name is Graham. However, this name cannot be considered canon, especially since there is already another Slytherin in the books named Graham Pritchard. The "new" name for Montague could be a mix-up.
The Werewolf Connection:
The story of Montague is an interesting cautionary tale of the way actions can come back to literally "bite you." If Fred and George had not shoved Montague into the Vanishing Cabinet that day in a moment of anger, he would not have been able to tell Draco what he learned about the other cabinet in Borgins, and Draco would not have let Death Eaters into the castle a year later, leading to Bill Weasley becoming part-werewolf thanks to Fenrir Greyback. Even more interestingly, this story is juxtaposed and entertwined with "Snape's Worst Memory," in which James and Sirius (counterparts to the Weasley Twins) bullied Snape "because he existed," causing him to lash out at Lily and lose her friendship, leading Snape to turn to the dark side and later give the prophecy to Voldemort, leading to the Potter's deaths and Sirius's imprisonment. The werewolf connection is in that story too, because Lupin's secret was one reason they bullied Snape, and Sirius had already played a prank on Snape trying to lure him to the Whomping Willow during the full moon, possibly to his death-by-werewolf. Sadly with the Twins and the Marauders, Gryffindor house pride against the Slytherins also didn't help anything.