Harry didn’t quite know how to describe in words the sense of kinship he felt with Professor Quirrell, except to say that the Defense Professor was the only clear-thinking person Harry had met in the wizarding world. Sooner or later everyone else started playing Quidditch, or not putting protective shells on their time machines, or thinking that Death was their friend. It didn’t matter how good their intentions were. Sooner or later, and usually sooner, they demonstrated that something deep inside their brain was confused. Everyone except Professor Quirrell. It was a bond that went beyond anything of debts owed, or even anything of personal liking, that the two of them were alone in the wizarding world. And if the Defense Professor occasionally seemed a little scary or a little Dark, well, that was just the same thing some people said about Harry.
One of the many Harry=Quirrell passages.