Harry Potter and the Deathly Hallows
Professor Minerva McGonagall
Affiliation: Order of the Phoenix
House: Head of Gryffindor
Professor: Transfiguration, Deputy Headmistress
Though Dumbledore is gone and Death Eaters essentially run Hogwarts, McGonagall stays at the school and does her best to protect the students. In Deathly Hallows, McGonagall shows her truly sassy and sarcastic side in her interactions with Snape and the stupid, cruel, and cowardly Carrow siblings that she's left with at Hogwarts. And it turns out that she's not as much of a total rule-follower as we thought she was – she even uses the Imperius curse on Amycus! And her duel against Snape is pretty awesome, too.
McGonagall also quickly recognizes that Harry is on an important mission, and figures out how to help. She organizes the other teachers to secure the school against Voldemort in order to buy Harry time to find the last Horcrux, the diadem of Ravenclaw. And she steps up and acts as the Headmistress of Hogwarts and the leader of the defense of Hogwarts. Dumbledore would be proud.