Professor
Remus Lupin
A shabby, excellent Defense teacher for one year. A werewolf, a friend of Harry’s parents, and a man practiced at leaving before he is asked to.
Lupin is the professor who gives Harry a way to fight despair and gives the class a boggart and a grade. He is poor, kind, and hiding a monthly transformation. Wolfsbane makes the hiding possible. Prejudice makes the job temporary.
He was one of the four students behind the castle map, with James, Sirius, and the friend who betrayed them. That sentence is a third-book answer. The stacks entry on the map stays vaguer on purpose.
Later in the story
His later marriage and the war’s cost to him belong to the last books. The teacher in the patched robes is the portrait to meet first.