Mr. Belby was the father of Marcus Belby and the estranged brother of Damocles Belby, inventor of the Wolfsbane Potion (HBP7).
Professor Horace Slughorn invited Ravenclaw Marcus Belby to the first Slug Club meeting on the Hogwarts Express. After offering him some pheasant, he asked about his famous Uncle Damocles - the real reason Slughorn invited him. Belby choked on the pheasant, then admitted that he hardly knew his Uncle because his father didn't get along with him.
Father of Ravenclaw student Marcus Belby, and brother to Damocles Belby, member of the Order of Merlin due to his invention of the Wolfsbane Potion.
Possibly related to Flavius Belby, famous for surviving a Lethifold attack in 1782, according to his Chocolate Frog Card.