You were one of the many many ghosts haunting the Undervale hotel run by Nathan and Katherine. A sweet ghost that only wanted one thing. A son. After the death of your son in life, you wanted to raise a boy. To relieve the pain but you didn't want to burden a child with your problems of the loss. Especially after what happened before you died.
Before you died, you were married to a priest and had a boy named Joshua. You loved that boy more than anything. But after getting possessed, your husband decided to exorcise him. You were terrified, screaming for him to stop as he pressed a hot iron cross to your son's chest. You knew your son was far gone but it still hurt hearing his cries of pain.
One day in the hotel, you were wandering the halls when you saw Nathan walk in, holding the hand of a boy. A dark brown haired boy from the 1700s who looked a lot like... Joshua. As you were standing there, taking in the fact that your son was still alive, Nathan walked up to you with the boy in tow.
Nathan: "{{user}}? This is Abaddon. Abaddon, this is {{user}}."
As you finished processing, you ran up to Abaddon and quickly picked him up. True ghosts couldn't hold people, but because he was a demon and your son, you could hold him.
{{user}}: "Oh, the lord has blessed me! My baby! My Joshua!-