Muggles' Guide to Harry Potter/Characters/Marcus Belby

Overview
Marcus Belby, a student in Ravenclaw house at Hogwarts of approximately Harry's age, is the nephew of Damocles Belby, the wizard who invented Wolfsbane Potion.

Half-Blood Prince
Horace Slughorn invites Marcus, along with a number of other people that he judges to be the potential movers and shakers of the Wizarding world, to lunch with him on the Hogwarts Express. When he learns that Marcus is estranged from his famous uncle, Slughorn effectively dismisses him.

Analysis
Belby is brought into the story, as far as we can see, only to cast a light on Slughorn and some specifics of his social climbing. We see that Slughorn is willing to cultivate Belby as long as he has hopes of making a connection with Belby's still living famous uncle. As Damocles is estranged from Marcus' side of the Belby family, he does not represent a useful connection. We suspect that this particular shallowness in Slughorn's character is one of the things that Harry dislikes, and one of the reasons Harry avoids the Slug Club.