The Dark Energy that she sensed had the quality that was only found in the Devil. It was not possible even for the strongest Devil Contractors to have that kind of Dark Energy, and therefore Santess Amelia was certain that the boy was a Devil.
"Calm down!"
Seeing her friend panic, Professor Mia immediately tried to calm her down. She realized that Amelia was scared because she sensed Dark Energy from Zero.
She knew that Amelia might find out that Zero could wield Dark Energy. But she didn't expect her to react like this.
"You want me to heal a Devil? Are you crazy? Instead of healing the Devil, Healing Magic would only do more damage to the Devil."
The Devil was hated by God, at least that is what is known. Maybe that's why healing spells do damage rather than heal them.
Elves, dwarfs, and humans were all entitled to be healed by healing spells but Demons were not.