Erica Lennox was the hidden villainess from Person of Interest episode 4.04, "Brotherhood."
She was played by Rosie Benton.
Erica Lennox was a DEA Agent on the scene of a drug deal that became a murder scene, which is where she encountered John Reese. At the precinct, Lennox informed Reese about concerns of a mole in DEA working for The Brotherhood, and later showed video of youths Malcolm and Tracie Booker, who stole $500,000 in drug money.
Lennox and Reese rescued Malcolm and Tracie from Brotherhood members and took them to a safe house. While there, Lennox planted a phone in Tracie's possession, which belonged to Brotherhood leader Dominic, and left the house claiming that she called for backup and they're on their way. The ruse was discovered when Fusco informed Reese of the phone and called it while it was in Tracie's possession, revealing Lennox as the villainous mole. The "backup" that Lennox mentioned was actually Brotherhood members closing in on them, while the evil Lennox escaped with the drug money.
In the end of the episode, Lennox was captured by Link, and placed in a car with him and another member. Lennox demanded to speak to their boss, but the second member stated that she was, revealing himself as Dominic and killing Lennox with a shot to her head.