Check it out here: http://fangirlish.com/supernatural-10-x-4-recap-paper-moon/
“A recent string of werewolf attacks points to a surprising culprit – Kate (guest star Brit Sheridan), the werewolf Sam (Jared Padalecki) and Dean (Jensen Ackles) let live.”
The last time we saw Kate was in Bitten (season 8, episode 4). She was the lone survivor of three werewolves. Her boyfriend, Michael was killed by their best friend Brian. She, in turn, had to kill him. The last time we saw her was when she was pleading, through a recorded message, that Sam and Dean left her alone. She promised she wouldn’t kill humans. That she just wanted to live.