A princess as a playable RPG character is nothing new. In the entire Breath of Fire series, you got a princess as your female sidekick for example. Princes are a bit rarer, but how about a monarch who is actually a playable character?

It happens more than you think. I should also note that some monarchs are crowned king/queen during the events of the game, and with some of them, this is sometimes their royal status is not even revealed up to a certain moment in the scenario, so before you read on I must give you this

King Albert - The Legend of Dragoon

I never finished Legend of Dragoon, as I hate that game, but the fact remains that Albert is a playable character who joins the group as a playable character and has a wind spirit allowing him to transform into the wind dragoon. I cannot tell much about him though, due to my very little knowledge of this game.

Queen Audrey Zecksen Slaith - Atelier Iris 2

Not only is Audrey only crowned queen during the events of the game, she was not even aware of her own name, let alone her own royal status until her brother, prince Maxillian, also known as Max, reveals it. Until that time she believes to be an orphan named Fee, adopted by Althena Church. When she learns of her heritage and her position in the Slaith Dynasty, she takes it well, and when Max is not capable of leading the Simsilt Liberation Army, who fights against evil consul Theodore of the Silveresta Empira, Fee will take over the lead and take her place in the Riesefeld, as Queen of the Slaith Dynasty. However when Felt, who fought alongside her, disappears, she adopts the persona of Fee once more in order to look for him, and will therefore be a playable character again, still like it or not, she is from that moment a queen in your group.

King Edgar - Final Fantasy VI

Edgar may be in the first place a very annoying womanizer, trying to flirt with anybody who happens to be female (including the defacto main character Terra), still he happens to be king. He is a playable character and famous for the machine tools he can use in combat. His brother is also in the party, the martial arts artist Sabin, making Sabin technically a prince even though he doesn't behave like one, and doesn't even seem to care at all about his royal status.

Queen Garnet Til Alexandria - Final Fantasy IX

Garnet starts the game as a kind of runaway princess who voluntarily allows herself to be abducted by Tantalus (she even requests Tantalus to kidnap her, herself, unaware that Tantalus was already planning to do this, for her own safety), a group of actors and rogues from Lindblum, and she develops a great friendship with most of the prominent members (in the end, although she won't admit it for a long time, even falling in love with Zidane, who is FF9's main character). Now in Garnet's case I do not want to speak of a Stockholm Syndrome (although he never uses the term, but it's clear Steiner believes it to be so), since Garnet was abducted not for criminal intend but for her own safety, as her mother, Queen Brahne is clearly on a rampage. After her mother gets herself killed by Kuja, Garnet will therefore take her mother's place as queen, but still remain a playable character who goes under the nickname "Dagger" in order to cover her true identity.

Now Garnet is even a strange entry here, but there is no denying she is a princess and eventually a queen. Point is however that she's not of royal blood. She is not even the real Garnet. The real Garnet died at an extremely young age. Dagger is actually a girl who had to flee her hometown of Madain Sarai because it was destroyed. She is not even human, however the only way you could see that on the outside was due to a small horn she had on her forehead. She was however the spitting image of Garnet who just died, and the king (still alive back then, as Brahne only went on the rampage after her husband's death) ordered that horn to be surgically removed so this refugee girl, could take over Garnet's identity, and therefore also eventually become queen.
What her real name is, is unknown. Some fans believe it to be Sarah, but this was neither confirmed nor denied.

King Roan - Grandia II

Roan just appears to be a traveling kid, but has been concealing his true identity as crown prince of the Cyrum Kingdom for a large part of the game. Once his father appears to have fallen from some tricks played by Melfice, the brother of main character Ryudo, who is possessed by the evil entity Valmar, Roan reveals his status as prince, and he then takes in his father's place as king. Initially he leaves the party, however for the last two dungeons he's be back, even with a new dress-up, and even with a few stronger abilities. Now in Dutch I often call him "prins Peuter" before he's crowned king, and "koning Kleuter" after, but it won't do any good to even try to translate that joke to English.

Queen Shanda - The secrets of Dyrt (original, .NET version)

Queen Shanda is completely shrouded in mystery, and the heroes of the game will even doubt for a long time if she's an actual queen or just some stupid clown playing a joke on them. I can reveal though that she's actually a real queen, even though her kingdom has been erased from existence. Shanda is only a guest character and will only join the party for one dungeon, but fact remains that she is a playable character, and that she is a queen, thus a monarch, and thus fitting for this list. Shanda is mute and therefore she cannot speak. The reason why will be revealed later, along with the reason why she is still alive after many thousands of years even still looking quite young. Although she only appears to be a kind of "start of game" prank at first, she will later on prove to be the key to some very important mysteries eventually leading to their solutions. It's therefore important you take good notice of her when you play The Secrets of Dyrt.