I haven't played the game in several updates, so I could be wrong, but last I played they all give it to you pretty much without a fight. But the one that's closest to a challenge is Rinny, which is pretty much just some extra dialogue.
I don't think there is anyone. Again, I could be wrong because last I played was *several* updates ago (I almost forgot if if it was Rinny or Rinni) but to my knowledge, every character you can give the key to just gives it back immediately. There *could* have been a change or a new, more punishing character added to the list of candidates, but I doubt it.