You have to wonder, since there's a Queen of Light and a Queen of Shadows both reflecting Helena's mother and a Princess of Shadows to reflect Helena....why there is no Princess of Light as well? Then you realize that's what Helena herself is!
Think on it, what would the Princess of the Good Queen do if her world was in danger? She would try her best to save it. She would be kind, clever, brave, and willing to associate even with a seemingly skeevy dude like Valentine. Reflecting an opposite personality to the Dark Princess....just like Helena does.
In addition, the clothes she wears during the dream adventure/most of the movie. White/ extremely light colored and loose fitting clothing. At one point when Helena is talking to the Light Queen/her mother in the Border Lands you see the Queen in a similar outfit briefly, humble and loose fitting white pants and shirt. She also wears no mask like the Light Queen does, reflecting that she is meant to use the Mirror Mask charm - something to be worn only when needed.
The Prime Minister is literally see-through. Combined with the fact that he has less of his face covered than any other character, this may be because he's one of the top politicians in the land, and therefore needs to be transparent in the metaphorical sense for his people.
