Alien, but he's coming to understand it through Harry.

He often specifically asks about friendship, about how Harry feels towards others, and particularly Hermione (as Harry's clear #1 friend). And when he asks, he contrasts his own feelings and experience to Harry's.

The Defense Professor ate several spoonfuls of soup in silence, then; and when he spoke again, his voice was oddly flat. “You really care about that girl.”

" I suppose that is why she was able to bring you out of your Dementation?"

“I did not have any friends like that when I was young.” Still the same emotionless voice. “What would have become of you, I wonder, if you had been alone?”

“Then here is what I might have done at your age, if there had been anyone to do it for—”

And

“It is a very strange thing,” the Defense Professor said, his voice now soft again, almost inaudible. The man was not looking at Harry, and Harry saw only his back. “A very strange thing... There was a time when I would have sacrificed a finger from my wand hand, to work upon the bullies of Hogwarts as we have worked upon them this day. To make them fear me as they now fear you, to have the deference of all the students and the adoration of many, I would have given my finger for that. You have everything now that I wanted then. All that I know of human nature says that I should hate you. And yet I do not. It is a very strange thing.”

Compare to, where Quirrell's voice is "filled with some indecipherable emotion."

“You truly do care about that girl,” the man’s dim outline said softly. “You care about her in the way that none of them are capable of caring for their own lives, let alone each other.” The Defense Professor’s voice had become strange, filled with some indecipherable emotion. “I do not understand it, but I know the lengths you will go to because of it. You will challenge death itself, for her. Nothing will sway you from that.”

IMO, it's unlikely that EY makes Quirrell an unqualified monster. EY, like Harry, wants to save everyone. Even a psychopath has some feelings - is not a completely alien Clippy.

I hadn't considered it before, but perhaps that is what happened to Quirrell - like Harry, extremely susceptible to Dementors, but unlike Harry, with no friends who might have pulled him out of his dementation.

“What would have become of you, I wonder, if you had been alone?”