Short I know but I have to update another story plus if you like fairy tail check out my other story Celestial Keys. Its on a different account though
____________________________________
20 years before Luna....................

"William stop being upset come and pet my cat ears.""
I'm sorry that girl left you I still don't understand how a girl could leave you." Luna replied cheerfully if only she knew she was the girl that left him.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"William I'm sorry but in order to protect you I shall go make a contract with Lucifer and for the price of my soul I shall ask him to erase my mind of the last year." Luna the reaper says.

The tears of Luna slowly cascade down her face staring at her lover on his death bed.

"I love you William but I can't see you suffer even if it means forgetting the love I've had with you and losing my soul!"

Silence followed as Luna jumped out the window carrying her reaping tool and the book to summon King Lucifer.
~~~~~~~~~~~~~~~~~~~~~~~~~~~
The king of hell looked at the shinagami girl.
Carrying her reaper staff begging for a contract with the purest soul he had ever seen.
That soul was mouth watering filled with love,pain, and the best part guilt.

"Alright I'll sign the contract with you what are the terms of the contract."

"Lucifer I want you to erase the past years of my life including the one I love and cure his sickness and turn me into a demon where I still have your mark but you shall order me instead. Those are the terms of our contract.
The King stared at the reaper it was very rare for a shinagami to have such soul as she did.
"Alright I accept the terms of the contract where do you want your mark."
"Can I have it on my stomach?"
"Sure!"
Luna's screams surrounded the area. When it was finally done the king stared at the new demon.
"I shall also give you a demon blade since you had such a tasty soul. Maybe since the crow went to find a soul I'll make you my sons guard.
________________________________
20 years later....,......................
William watched the female from afar as she was no longer to visit or see him. He had discovered he was healed and went to go see Luna but found her body in her house on the floor. When she finally awoke her eyes were red and stared at him with a derp face. She knew nothing of their romance or her sacrifice only that she was best friends with William T. Spears.
And that's how come he hates demons.