Have your character take up genealogy as a hobby with the fervor of your aunt who just discovered ancestry.com. Have them make ridiculous comments or claims in threads relating to their new "expertise," such as telling a WC man he's the last living descendent of a deposed foreign Duke, or writing a short treatise for the DP on how [powerful pureblood family] are all descended from a very Muggle Knight of Arthur's round table and cause a scandal. At some point, this character should relate an ancestral crime as an amusing anecdote ("did you know, the only reason your last name has that French prefix is because your great great grandfather kidnapped, raped, and married a Frenchwoman? It was a family secret until it became vogue to have French blood.") and be challenged to a duel by the offended party, who wants to defend their family's honor.

At least 60% of the claims this character makes should be bullshit.