Kat Russo was an FBI agent when she was shot and seriously wounded in the line of duty. Her fiance at the time, Robert Jessup, turned her into a vampire. She had no previous knowledge that Robert had been a vampire when he asked her to marry him. She hadn't wanted to become a vampire and blamed him, which is probably what caused the split between them. (Identity Crisis)
Kat comes into The Gates looking for a suspect in the investigation, telling them that Robert Jessup was the suspect. She lies to Nick Monohan, Leigh Turner and Marcus Jordan about the real reason why she was going after Robert. She arrests Nick when she spies him trying to hide evidence from her, to protect Dylan and Claire Radcliff and the other vampires in The Gates. Once Russo had the flash drive from Jessup's account in the Red River Bank that had a list of at least 10,000 names that he had helped, she intended to leave. However on her way back to the hotel, she sees that Robert is back home and goes in to confront him. Russo starts a fight with Robert - her revenge on him turning her into a vampire. She wins, by pushing him into an exposed stake-like piece of wood on the wall that went through his heart, killing him. Now that Russo was out of the office, Nick makes Leigh and Marcus investigate Kat and because of this, they learn that Kat and Robert Jessup had been engaged and that Kat was considered dead as far the FBI knows. Kat confronts Nick and asks for the real flash drive, which he gives her, but warns her that if she takes it he wouldn't be able to protect her anymore. She ignores the warning and leaves with the flash drive. Unbeknownst to her, she is followed out of The Gates by Ben McAllister and two vampires. (Identity Crisis) Her eventual fate is left unknown.