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?”

[…Hero?]

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.”

[…What?]

I will become strong once again.