Queen of Hearts

The Queen of Hearts is an anonymous character in Pretty Little Liars. He/she is most likely part of the A-Team or even the leader. She/He first appeared in the Season 3 Halloween episode "This Is A Dark Ride" she/he is the reason for Aria being drugged and also tried to kill Spencer and Paige.

Season 3
In "Kingdom of the Blind" Mona is seen at the end singing "The Eye Abides the Heart" and is holding a queen of hearts card, meaning that she may be in cahoots with her/him.

In "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. He/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.

At the end of Mona-Mania 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)

In "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 Calebs help to figure out who's credit card it was.

In "A Dangerous GAme" the identity of the Queen of Hearts will finally be revealed.