This morning Star Wars officially made public the identity of the Star Wars: The Force Awakens character TR-8R. "Traitor!" He's a mad one. As suspected, this stormtrooper goes by the name FN-2199, one of three troopers revealed in the book "Before the Awakening", a set of short stories giving background on the three main characters in the film. That story was about FN-2187, aka Finn. This character knew Finn well before the awakening, hence the knowledge of his being a traitor when he finds him on Takodana, near Maz's castle.