Remus John Lupin, Was Harry's Defence against the dark arts teacher in his 3rd Year.

He Taught Harry How to Fight Off Dementor's , Using the Expecto Patronum

Remus Lupin: What Thought did you choose, Harry?

Harry: The First Time I Rode a Broom.

Remus Lupin: That's Not Good Enough, Not nearly good enough

Remus Was also one of the Four Marauders, Known as Moony.

At First Harry Thought Remus Had betrayed him, By Helping Sirius.

Then He finds Sirius Is Innocent.