Susan Darcy is a disgraced federal agent assigned to the Panzer case. She first appeared in Blinking Red Light and has been a recurring character throughout Season 4. However, after the botched FBI sting operation to catch Red John which led to the death of CBI Luther Wainwright, Darcy's duty was taken over by her supervisor Alexa Shultz.
In Blinking Red Light, she becomes the lead investigator on the Panzer case. Then in Always Bet On Red she quickly becomes suspicious of the Jane's claim that Red John is dead. He is in fact alive, something Jane doesn't want her to find out, as Red John would kill her for having such knowledge.
In that same episode Red John is able to follow her home and take a video of her sending it to CBI headquarters on Grace's computer with a text saying "she's cute I'm going to have fun", but Grace and Jane make it to her apartment in time preventing her from possibly being killed by Red John,
In the end Jane is able to trick her that Red John is dead, but she nevertheless continues investigating and is even more suspicious when a morgue attendant, who she previously talked to was dead after arriving with several CBI authorities to Rosalind Harker's house where Red John visited but escaped before authorities arrived.
In the episode Cheap Burgundy, she had Jane assist her in a case, all the while trying to find out if Red John was still alive. Obviously, Jane denied it. During one of their conversations it was confirmed that Susan had lost her sister to drowning and that she blames herself for what happened to her.
Eventually, Jane admitted that Red John was alive and before catching the killer she accused Jane of being one of Red John's accomplices and at the end of the episode she told Wainwright.
In the Season 4 final episode The Crimson Hat, Red John reappears and admits that he has a good friend in the FBI who found out that Jane didn't kill Lisbon, who disappears, and Rigsby, whose supposedly body is found but unidentifiable, in order to become his friend. Together with her team, she interferes an operation of Lisbon's team and gets Jane in trouble. As the limousine is about to get away, she shoots for the car. When she opens the door, she sees Luther Wainwright on the backseat and discovers that she killed him.
In Season 5 premiere, Alexa Shultz mentioned that Darcy is suffering from a mental breakdown following the botched FBI sting operation in which she was responsible. As a result, her assignment to catch Red John came to a halt. According to Agent Reede Smith, her career was destroyed. It is possible that Red John needed to kill Wainwright because he could have fingered Darcy as the one person on the FBI he told that Jane might not have killed his team, as he suspected something by the way Lisbon was acting.