The Debt-Ridden Demon King Works for the Celestial Realm

After accidentally landing himself in an enormous debt, the Demon King had no choice but to sell himself to the evil Michael to keep the Heavenly Realm from flattening the Demon Realm.

"What?! You want me to work for you? No way, I'm not doing it!"

"We'll cut your debt by twenty percent."

"When do we start?"

To pay off what he owed, he had to take on an angel's body and travel between planes, helping the heroes there defeat the Demon Kings of their respective worlds.

But, well, these guys are so pathetically weak that she could take them out in one hit, couldn't she!?

This novel is a slow burn. The protagonist becomes invincible starting in Volume Two.