Armando Dippet served as Hogwarts headmaster immediately before Albus Dumbledore in J.K. Rowling’s fictional history. Dippet is identified as headmaster during Tom Riddle’s years at Hogwarts.
In Harry Potter and the Chamber of Secrets, a memory of Dippet appears through a magical diary connected to Tom Riddle. Riddle asks Dippet for permission to remain at Hogwarts over the summer because he has nowhere else to go, but Dippet refuses because the school is closing for the holidays.
Dumbledore succeeded Dippet as headmaster before the events of the main Harry Potter story. The succession matters to the series’ backstory because Dippet was in office when the Chamber of Secrets was opened and a student, Myrtle Warren, was killed.