Dark Guardian

"I was going to make a bet on whether your soul would be corrupted."
"When you become an adult."

The demon "Luce", also known as Lucifer, Satan, the King of Hell, etc., plans a bet with God over Elijah, who possesses a rare pure soul.
On the day he meets Elijah for the first time, Luce tries to give up the bet after Elijah discovers his true identity, but Elijah holds him back.

"I just have to choose between him and you, don't I?"
"Don't give up on me already."

Luce is impressed by Elijah's defiance and agrees to take him under his wing.
He visits Elijah from time to time and insists that he sign a contract with him when he comes of age.

The day Elijah finally becomes an adult, He makes an unexpected request to Luce...