|"Are you a wizard or not?"
The title of this article is conjectural. Although it is based on canonical information, the actual name is conjecture and may be supplanted at any time by additional information released from canonical sources. If this occurs, please move this page to the appropriate title.
Mr. Lupin was the father of Remus Lupin. At some point during the 1960s or early 1970s, he offended the werewolf Fenrir Greyback, which lead Greyback to bite and infect his young son in revenge, which is how Remus Lupin became a werewolf. 
Along with his wife, Mr. Lupin tried all manner of treatments for his son's lycanthropy, but in those days, before Damocles's invention of the Wolfsbane Potion, there were no cures that actually worked. He and his wife worried that their son would not be able to attend Hogwarts School of Witchcraft and Wizardry, due to his condition, but Albus Dumbledore made a special provision for the Lupin.
As Remus Lupin is known to have been a half-blood, it stands to reason that at least one of his parents was magical. The fact that Mr. Lupin angered a werewolf indicates that he had contact with the wizarding world — implying that he may have been a wizard. However, it is possible that Remus Lupin's mother was not a muggle, but rather a witch of muggle parentage.
There are hints that Mr. Lupin is deceased, for example, the fact that it was James who financially helped Remus after Hogwarts, not his parents. This could also be explained by he and his wife having some sort of financial problem, even though both of these statements are mere speculations.
His name might have been John because his son Remus' middle name is John and Rowling has a habit of naming her characters after a relative of theirs, for example William Arthur Weasley was given the middle name "Arthur" after his father, Ginevra Molly Potter (née Weasley) was given the middle name "Molly" after her mother, Ted Remus Lupin was given his first name "Ted" after his maternal grandfather and his middle name "Remus" after his own father, Ronald Bilius Weasley was given the middle name "Bilius" after his uncle. Also Harry James Potter has his father's name as his middle name and Albus Percival Wulfric Brian Dumbledore has his father's and possible several grandfathers forenames in his middle names.
- Harry Potter and the Prisoner of Azkaban (Mentioned only)
- Harry Potter and the Half-Blood Prince (Mentioned only)