Ho -

I have a little argument with one of my friends:

 

It's about the question if Unversed can attack Kairi and other princesses of hearts?

I know that "darkness" is no thread to them but are they (Unversed) exactly that kind of "darkness"?

Darkness is hardly no threat to them. They're personally unaffected by darkness and so they can use corridors of darkness and their hearts cannot turn into heartless. However, a heartless could attack them and steal their heart that way. Eg. Ansem, seeker of Darkness, who created the Keyblade of People's hearts using their hearts.

Unversed are creatures created from Vanitas and are basically extensions of himself. They focus on more as being the different type of negative emotions than the darkness as a whole, like heartless do.

So, Unversed and Heartless both pose physical threats to the PoH, but they cannot become heartlesses and nobody can become an Unversed, because they're simply apart of Vanitas. Nobody else can create them.