Her Devil King

He is the devil king and she is just an ordinary witch, he is feared and respected by everyone and she is loved and adored by everyone. People remember him for his unmerciful punishments and the chilling wars he fought. She is remembered for her kindness and good deeds. They are so different in everything, from the species to the food they eat then how are they connected?
Are they mates? But he already has a mate.
Meet Aiza Elsie Cromwell and Quentin Roux Adler
Aiza is a 20-year-old beautiful and charismatic witch who lives with her parents, grandmother, and siblings. She is very empathetic and kind, as a witch she has the powers to heal people. Cromwells are the only ones who are blessed with healing powers which they got from their ancestors. Aiza loves to teach the village children about the powers and everything which a human being in case a witch should know. Everyone in the village refers to Aiza as a goody-two-shoes but no one knows the secret she is hiding from everyone.
Quentin Roux Adler rules the earth and hell because he is the devil of hell and king of earth and he is called devil king. Pandora is the kingdom which he rules, small villages are scattered around his kingdom. He may be the devil but he knows how to rule a kingdom and his people. He is many years old and no one knows his exact age and no one dares to ask him that. He goes by his rules and wants his people to do the same. If someone dares to disobey his rules then that person could be thrown into the deepest pits of hell.
How do a devil king and a witch meet? What is their connection?