Agent Erica Lennox was a DEA agent and a mole within law enforcement working for the Brotherhood.
Lennox graduated from Annapolis, Maryland, at the top of her class, and served two U.S. Navy tours before joining the DEA. At an unknown point in time, she began working with the local street gang the Brotherhood, being their source inside law enforcement. She was unaware of the identity of Dominic, their mysterious leader.
Lennox was present at the scene of a drug deal turned shootout, with Dominic inconspicuously being one of the survivors. She met Reese at the scene and later talked with him at the precinct, suspicious of his missing history prior to joining the NYPD. Reese demonstrated his knowledge on her background in turn, and prompted them to discuss the case - Lennox showed Reese footage of missing siblings Malcolm and Tracie Booker, who were on the run after stealing drug money from the crime scene. She also informed Reese of the rumor concerning the mole.
Lennox later rescued Reese and the kids from a Brotherhood attack and came with them to a safe house, where she attempted to sympathize with the kids and establish a sense of long-desired reliability. She left after calling backup, but planted a phone on Tracie that Fusco believed to belong to Dominic - when they called the phone and learned that Lennox gave Tracie the phone, Reese and Fusco realized that Lennox was the mole. Finch theorized that Lennox wanted to keep the money for herself. Lennox was able to retrieve the money and presumably vanish.
High-ranking Brotherhood foot soldier Link was able to locate Lennox and showed her to Dominic, who he picked up from a laundromat as a result of a deal with Shaw. When Lennox asked to meet their boss (implying she didn't know Dominic's identity), Dominic shot her in the head and then said she just met him; they dump the body on the street.