Who to bring along in the Ravenloft heist?

Hey all, im preparing a bunch of different quests and locations in my docs for when our party starts up again. The party will be tasked to steal argynvosts skull among some other shenanigans in the castle. I intend for them to have the opportunity to fight the brides as well when they are there. I want them to have an ally in the castle but Im not sure who. Their fated ally isnt with them at the moment. Should I go Van Richten? Doru? Ezmerelda? Any fun ideas?

So all in all. Need help picking a character to accompany the party in a Castle Ravenloft heist