Kain could not have possibly been more stunned if he tried. When Vader had first spoken up, the dragoon had not quite believed his ears, so Mitch's voicing of what everyone had likely been thinking at that moment was much appreciated. The fact that the Sith Lord had just boldly announced that he had an apprentice in Niflheim in front of all of Asgard was not something that he would have ever imagined him doing, especially considering that Kain knew little about him.
When he mentioned the name of his apprentice, Kain's stomach dropped to somewhere around his boots and his face twisted into something that seemed equal parts shock and rage.
This man was apprenticing the dragon?! And unless "apprenticing" meant something else on his world, which it certainly didn't seem to from the way he spoke of her, that meant he was teaching her to hone her powers.
It was at this point that Kain was rather violently reminded of something he had learned long ago in Baron's court: diplomacy was hard when you were faced with people that made you want to lunge across the table and beat them half to death.
"Are you insane?" Kain asked, his voice deathly calm and cold as his features settled into a glare. "You are training one of our enemies. What were you even thinking?"
He could hardly fathom that anyone on his team would be stupid enough to hand Niflheim some extra knowledge, as though they weren't doing a well enough job of keeping them on their toes already.