Okay, so here we're going to have a clear victim. The other royal person is the guy being lied to and deceived, and he doesn't even get the dignity of a consistent name. He's called "a third person, who shall be nameless" (26), "the other exalted personage" (28), and "the third personage" (28).
Great. Way to give us something to work with, Poe.
So, the most obvious answer here is that the guy is the king, husband to the queen, and thus supposed to represent kings (either of France or other countries) and kingship in general. Although it seems just as likely that this king is playing the role of the king in various games. If you know the rules of chess, then you know that the king is simultaneous the most important and (for most of the game) the weakest piece. He can't move very far or fast, but the game is over when he's caught.
We might think of Poe's king in the same way. He may seem oblivious and ineffectual, but the whole game began to keep him from finding out about the letter. If he does—game over.
Whoever this king is, he seems pretty oblivious to what's going on around him, including the fact that his wife is lying to him. There's also the suggestion that he doesn't have much power, or, if he does, it's being usurped by the queen.
But the whole world seems to be conspiring to deceive him. First, the queen receives secret letters. Second, D— makes sure that he doesn't put the queen's funny behavior together with the letter lying on the table. Third, G— and then Dupin work together to retrieve the letter and keep him from uncovering the secret.
So, maybe he's oblivious; maybe he's really busy; maybe he just trusts his wife. (Shocking, right?) Apparently he's not actually running the country, because D— has been using his power over the queen "for some months past" for "political purposes to a very dangerous extent" (30). Does this mean that the queen is the main person in charge? That she has all the power to make the political changes D— desires? Or, is D—making her do work in service of a rival country, or dangerous faction within the country? Whatever it is, it has gone on for a year and a half. All this time the king hasn't found out what's going on.
Dupin would not be proud.
As Lacan points out, the king is in some ways D—'s double. In the first purloining scene, D— takes the letter without the king noticing. In the second purloining scene, Dupin takes the letter without D—noticing. In any case, the king is the story's most pronounced symbol of blindness.
So, here's one secret about the story: the searchers' goal isn't to reveal the truth but to continue to conceal it. And the king is the biggest victim here, although it's at least partly his own fault. Jacques Lacan compares him to the emperor in the children's story the "Emperor's New Clothes," whose advisers convince him that he's wearing fine clothes even though he's actually naked.
We could say that the king is the biggest loser in the game of "The Purloined Letter" because he's always farthest from the truth. Then again, is it possible to lose a game you don't even know you're playing?