Akuto Sai was left on the door steps of a church by a strange woman. He grows up and transfers into the Constant Academy for Magical Arts, wanting to be a high priest. He meets Junko Hattori on the train to the school, and forms a pact with her to make the world a better place. The predicting spirit Yata Garasu predicts that he will be a Demon Lord upon graduation. Yata's predictions have never been wrong, and everyone becomes afraid of him as a result. Even so, he still tries to do the right thing when situations happen around him and he is shown to have an incredible amount of power within him, shown when he absorbs a demon dog's mana to revert it back to normal, something which a normal mage is unable to do. He struggles from time to time when his power comes forth and attempts to use the destructive energies to help instead of harm. Sai aims to become part of his country's highest order of magicians and contribute to society.
Later, he learns to manage his evil powers, so he can use them on his own will. He normally uses them when he gets angry. When he reminisce his past better, he remembers that, when he was a child, he wanted to destroy the "Gods" -that actually are Giant Computer Systems-. When he was attacked by Junko, he carries her and tells her that after it was all over, he would take good care of her.