Harry Potter and the Prisoner of Azkaban
Professor Filius Flitwick
House: Head of Ravenclaw
Flitwick is the friendly and very tiny Charms professor who tells us about the Fidelius Charm, a major plot and thematic point. We hear about Flitwick in passing throughout the rest of the book, but it's his place at the table with McGonagall, Fudge, Hagrid, and Rosmerta during the Three Broomsticks revelation scene that stands out. That scene, and Flitwick's knowledge about the first war with Voldemort, shows that he's a trusted member of the wizarding elite, as well as an important member of the Hogwarts staff.