Which Hogwarts founder is the ancestor of Salazar Slytherin’s heir in J.K. Rowling’s books?

Salazar Slytherin is the Hogwarts founder identified as the ancestor of his own heir in J.K. Rowling’s books.

Slytherin was one of the four founders of Hogwarts School of Witchcraft and Wizardry. In Harry Potter and the Chamber of Secrets, the heir of Slytherin is revealed to be Tom Riddle, who later becomes Lord Voldemort. Riddle uses the Chamber of Secrets and the basilisk to attack students while pursuing Slytherin’s belief that Hogwarts should admit only students from magical families.

The phrase “heir of Slytherin” can be confused with the founder himself, but they are different people. The heir is a descendant, and the books identify Voldemort’s mother, Merope Gaunt, as part of the Gaunt family descended from Slytherin.