Credit goes to Burritokitten

When Yuri adopted Crescent, he developed what mortals called ‘Worry’ and ‘Concern’ due to Yveltals having the name ‘Death’ in their names.
Yuri was afraid at what they'd do to Crescent, even if he was a baby, was still a Yveltal and feared of being taken away, Perma Pamped or even worse.
He mostly kept Crescent at home, but since he often snuck out late at night, he has to find ways to tire Crescent put.

He often took Crescent on expeditions and explorations, and not to mystery dungeons, but tombs and ruins of his previous life in the Ancient Era.
He decided to often stuff Crescent down his diaper to hide him whether they left or returned. Also told Crescent to stay still in his diaper and to only come put when he says so.

Unknown to Yuri, this is what got Crescent into stuffing things down his own diaper for fun at parties. Crescent did often poke his head out just to see Yuri blush.

Yuri and Crescent belong to Me
Pokemon belong to Game Freak