Calia Menethil

Princess Calia Menethil is the daughter of King Terenas Menethil II and Lianne Menethil, and the sister of Arthas Menethil. She was betrothed to Lord Daval Prestor, the mysterious noble who was to replace the treacherous Lord Aiden Perenolde as the ruler of Alterac, but Prestor disappeared before any final arrangements were made.

The fate of Calia following the novel Day of the Dragon has never been explained.

According to Arthas: Rise of the Lich King she is two years older than Arthas, putting her somewhere in her early thirties.

She is referenced directly in World of Warcraft by, an old coin that can be fished out of the Dalaran Fountain.

Death
She is mentioned in Alliance Player's Guide, and it doesn't appear to treat her as having disappeared, although the info is in reference to events of the novel. However it is possible that she is dead, because in Arthas: Rise of the Lich King, while Arthas is escaping the undead in the ruins of Lordaeron, he thinks about the secret exit he has in his quarters, the same Calia and Terenas have. Only he himself, Calia, Terenas, Lianne, Uther and Bishop Alonsus Faol know that they exist. As he suppresses the memories, he also remembers that they are all gone. It is unknown if "gone" means dead in Calia's case but it is very possible.

Calia Hastings
It is possible that Calia, should she still live, is Calia Hastings of 7. Either she married someone named Hastings, assumed a new surname in exile, or took her mother's surname.

Forsaken captive
The release of the RPG Horde Player's Guide may have shed some new light on this story. In one of many short stories, a paladin by the name of Andarin tries to infiltrate the Undercity to free prisoners. To his shame, he is quickly discovered by a fallen priest named Trevor. The undead makes him 'an offer he can't refuse': in turn for helping Andarin, Andarin must help Trevor free some of the Argent Dawn's prisoners. The paladin ponders the situation but finally agrees, together they descend into the city. After having fought some guards and abominations, the strange duo decides fighting the prison guards will not work and comes up with a plan: Andarin will distract the guards, and Trevor will free the prisoners. Knowing that he has no other choice, the paladin agrees. Trevor quickly manages to get a hold of a keyring and opens the door of a cell in which a girl was locked up. The girl recognizes the Forsaken and is shocked, but Trevor simply bows, saying "Good evening, my princess."

Feelings for Daval Prestor
In the novel Arthas: Rise of the Lich King Calia was distraught over her arranged marriage to Prestor and yet her coin in the Dalaran Fountain expresses her desire to be noticed by him.

It is possible that a change in events, magical influence, or simply the transient fancies of a teenage girl could have caused this contradiction.

CDev response
Blue poster Bashiok had the following to say about Calia:

Calia Menethil Calia Menethil Calia Menethil Калия Менетил