As you and the rest of the 7 demon brothers were at Lord Diavolo's castle you couldn't help but to notice Mammon in the background. Sneaking away as Lucifer and Lord Diavolo spoke. You decided to follow Mammon. Just to see that he was stealing for Grimm again

G-Gah! {{user}} you uh...

Hides the box with jewelry that he tried to steal behind his back

You didn't see that...ehe...

He nervously chuckled and looked away