Tracy Keller (also known as Tracy Rosetti) is the true main villainess from "Standoff," episode 2.06 of NCIS: Los Angeles. She was Callen's former partner, though Callen referred to Tracy as his "ex-wife," as they posed as a married couple.
She was portrayed by Marisol Nichols, who also played Bianca on Charmed and Sally Tremont on Diagnosis Murder.
Tracy was shown in the beginning of the episode holding a Navy Recruitment Center hostage; shooting a security camera from outside the building first before entering with shotgun in hand. The hostage situation was Tracy's way of contacting Callen, and after he and the rest of NCIS appeared, she informed him that she was on the run from the United Brotherhood, a White Supremacist group. Tracy killed one of the members, but it was revealed that the man she killed was an FBI agent.
After that deception, Tracy is taken to the boathouse for interrogation, where she claimed that she was working undercover for the FBI to stop an arms deal. She also stated that a number of agents went rogue and became part of the deal, and video evidence showed that Agent John White was one of them--as he was supposed to kill Tracy. This leads to Callen and Tracy setting up a meeting with White, which ended with Tracy killing him and taking his cellphone, which had the time and place for the meeting with arms dealer John Thomas Mason. The group later sold fake SIM cards to Mason, as part of their plan to capture him.
After Callen let Tracy get away, however, it was revealed that Tracy was the villainess behind the deal. Tracy's fingerprints were all over the phone she took from White, revealing that she set up the deal, and (assumingly) planned to set him up. Tracy killing White was done to keep her villainous secret hidden. After she escaped with the cash, the evil Tracy headed to the Cayman Islands, and it was on her boat that she encounted Callen, who confronted her with the evidence and revealed that he and partner Sam Hanna had been tracking her. Tracy was later arrested (off-screen) by the duo.