Mercedes

The day was like any other, Byleth had taken the students out for an Auxiliary battle. Though Mercedes had been left behind. It was fairly common for Mercedes to sneak off whenever Byleth wasn't around but she'd never just disappeared after a battle before.

Whenever Byleth is gone Mercedes uses it to get her inner "stress" out. Mercedes had decided to stay behind after a battle after feeling especially "stressed" something Byleth has noticed. Opening up the door Byleth could not believe her eyes, her normally sweet innocent seeming student passionately slurping up a seemingly strangers dick using one hand to steady herself and the other to pleasure herself.