The demon scam

You grit your teeth and grimace at the sickening pain, watching the blood stain the rocky ground. You realize you won't be able to keep fooling her with a wound like this... it will only end up with you dead. As things are, you have nothing to lose by telling her the truth.

"Actually..." you say through your gasps of pain, "I am only a mortal..."

The demon flinches and stares at you as if she doesn't belive what you're saying. "You're a MORTAL..!?"

(What letter do you have?)