The mole in Citadel, who ended up bringing down the titular spy agency, eventually turned out to be none other than Mason Kane.
Through most of the series, we were led to believe that Nadia Sinh was the mole — not only did most of the characters think that was the case, but she did have an unexplained several-month disappearance just before the agency fell.
But no, episode 6 shows us that Kane in fact was the spy. He leaked Citadel's secrets to his mother Dahlia Archer, under the pretense that she was going to start a government inquest into the agency's illicit actions. He didn't know that she actually worked for Manticore, and used the information to hunt down Citadel's agents instead.