Bailey Markham was a Homeland Security agent and Greg Knox's former mentor.
Markham first appears in the season 3 finale. He and Greg listen to a recording Greg got from Carlos Ruiz, his off the books confidential informant. On the recording Tommy Egan confesses to doing the Felipe Lobos murder with Ghost and they hear him kill Ruiz. Greg is excited about finally having evidence against Egan and Ghost but Markham tells Greg the evidence is inadmissible. He then tells Greg to remember what he taught him: who do you want the most and who has the most to lose.
Markham reappears in episode 3 of season 4. Markham attends Greg's funeral and notices that Angela Valdes, Greg's ex-girlfriend, is there. He is questioned by John Mak and Cooper Saxe about possible information on the Knox murder. Markham claims Knox was not corrupt but they don't believe him. Markham later confronts Angela in her office and tries to convince her to confess she was Felipe Lobos' mole though she refuses because she claims she is innocent. Markham later meets Saxe alone and tells him about the Ruiz recording but never plays it for him. While in a bar Markham is visited by Mike Sandoval about teaming to clear Greg’s name. After Mike leaves Markham gets on his laptop and looks up a profile on Mike. It is shown he suspects Mike of being the mole.
In episode 4 of season 4 Markham gets called by Joe Proctor, Ghost's attorney, about teaming up to clear Ghost's and Greg's names. Markham plays the Ruiz recording to Joe in Joe's home and is later stabbed to death by Tommy who was secretly in the room. Proctor later disposes of the body and convinces Tommy to let him keep the recording which he hides in a P.O. box.