"I'm sorry it had to be this way, but when I want something, I'm getting it. You were a good help for the time being, but I'm afraid your usefulness has come to an end. You won't be king any longer.
~Mercer before stabbing Robb Stark in the chestMercer Frey was the former leader of the Thieves' Guild and the unconsummated son of Walder Frey, lord of The Twins and Dawnstar. He was banished from his homelands at a young age by house Tully for stealing thousands of valuable and precious items from around the Riverlands.
Mercer eventually went on to join the Thieves' Guild, eventually forming a deep bond, and in turn betraying and murdering the Guild's previous master Gallus, in order to secure a great deal of wealth for himself.
Mercer's extreme rage ended up being his demise, as, during a fight with Robb Stark, he accidentally got a vine caught around his neck and killed himself.