As a witch, Eva is very knowledgeable in medicine and has twice the life-span of a human. She has striking black hair, as only witches do, which always gives her away as one when not concealed.
Their knowledge of medicine was somewhat the witches undoing, as it it is intertangled with knowledge of poison, which people feared.
This is why, when she was young, she dyed her hair and pretended to be human. She even fell in love. But when he fell ill with a disease the humans did not know the cure for, she revealed her background to save him. She worked on the cure for fifty years, but he rejected her. After that trauma, she went into seclusion for many years.
This ended when the royal family asked her help to save the crown prince, in return for which they would grant her anything.
She demanded the sickly fith prince as "emergency rations" and raised him into a heathy young man.