Is It Really Reasonable for the Strongest Hero to Adopt a Filler Demon Lord?

"So, how about taking a look at me? I'm the Demon King, you know."

"Mm-hmm, I see you, little boy."

"So what do you want? You're the Hero and I'm the Demon King. We're mortal enemies! I'm off. Goodbye!"

But before the little Demon King could get away,

the Hero lady's jade-like hand had already caught him by the arm.

"Oh? And where do you think you're going? Don't forget, you're mine now~"

"I'm not! I didn't! Don't make things up! We're sworn enemies!"

"Oh? Where are we sworn enemies, then? In bed?"

"No, I didn't, what are you doing? Ow! Don't drag me! Hey, you, you, you, "

"You what? A little Demon King should be a good boy and listen, or your Hero big sister will have to punish you."

"Even Demon Kings have their dignity!"

"No. Not in front of me, you don't, "