The Queen of Hearts is an antagonist on Pretty Little Liars. An anonymous member of the "A-Team", whom attempts to kill Aria Montgomery on the Halloween Train by pushing her overboard and Spencer Hastings as well.
In "'A' Is for A-l-i-v-e", it is revealed that Melissa Hastings and Darren Wilden were the Queen of Hearts. Melissa drugged Aria, presumably, while Wilden attacked Spencer.
Queen of Hearts is the second disguise used by Melissa Hastings while doing work for the A-Team.