Queen of Hearts

The Queen of Hearts is identified to be Melissa Hastings and Darell Wilden in Pretty Little Liars.

Biography
Shim is most likely part of the A-Team. He first appeared in the Season 3 Halloween episode "This Is A Dark Ride" Shim is the reason for Aria being drugged and also tried to kill Spencer and Paige.

Pretty Dirty Secrets
According to a caption from a photo by ABC family, the Queen of Hearts first appeared here, watching Jason in the costume shop and wearing a different outfit.

This Is A Dark Ride

When Adam Lambert is walking away from Aria, we can spot the queen of hearts in the background looking at them. She is seen with the powder that Mona gave earlier in the episode. She then walks over and puts

the powder in Aria's drink.

Later, when Spencer goes back to look for Garrett, she has a dangerous encounter with the Queen of Hearts. She/He is trying to throw Spencer off the train. but she manages to run away. The Queen of Hearts chases her and starts to choke Spencer. But Paige comes to the rescue and she/he leaves a Red press-on nail in Spencer's hair. While Paige fights the Queen of Hearts, you could clearly hear male grunting voices, indicating that it is possibly a he.

Spencer meets up with Hanna and tests the nail on Hanna's nail. It's really big and Hanna says "That's a big girl," and Spencer replies, "Or maybe it's not for a girl at all." This leads many to question the gender of the Queen of Hearts.

Mona-Mania

At the end of the episode we see A burying the queen of hearts mask in the woods along with the Phantom of the Opera mask (which Mona was wearing, impersonating Caleb)

Hot Water

Paige tries to help the girls uncover the person in the Queen of Hearts costume. She contacts Shana, the girl who works in the shop, but cannot reveal that information. They stall her, and Emily looks at the costumer history and sends it to her phone. She says she will enlist Caleb's help to figure out who's credit card it was.

Appearances(2)
Season 3(2)
 * This Is A Dark Ride
 * Mona-Mania(Mask)
 * Pretty Dirty Secrets