There’s a Demon King Inside the Hero

I was summoned to another world, became a hero, and killed the Demon King. After fulfilling my duty and returning to my original world, what greeted me was an ordinary body, still holding the memories of my time as a hero. And then. “…The Demon King?” All that’s left in my mind is the lingering thought of the Demon King, my sworn enemy, who has now settled there. But I can’t give up like this. “Teach me magic, Demon King.” I will become strong once again.