Catherine Green, aka Boz (Sarah Smart), is the hidden main villainess from "Body of Evidence," episode 9.05 of New Tricks (airdate 24 September 2012). She is as a college librarian and the revealed ex-lover of Martin Longthorn.
The episode had UCOS investigating the death of a man named Christopher Smith (the first official UCOS case for new member Steve McAndrew, who replaced the retired Jack Halford), only for Christopher Smith to be revealed as Martin Longthorn. Catherine was interviewed by DSI Sandra Pullman due to the fact that she was the last person to see Martin alive, with Catherine revealing that she and Martin went on a date that ended when Martin informed her that something came up at work. Martin was revealed to have died of a catastrophic subarachnoid hemorrhage caused by a pre-existing aneurysm, leading to Sandra asking Catherine if Martin complained about having migraines, to which Catherine stated that he didn't.
With the help of Xander Levine from e-crime, UCOS learned that Martin was a member of Roguenet, a group of online political activists who hack into the computer systems of (among others) banks and insurance companies. A later look actually revealed that Martin was working to capture and expose the identity of Boz, the leader of Roguenet, who was eventually revealed as Catherine after a photo of her with Martin was uncovered, along with the fact that Catherine was his only date after it was revealed that Boz was female.
Catherine was interrogated by Sandra and Brian Lane, expressing defiance and denial of her true role, which was fully exposed afterwards by Xander when he got her password. In addition, the evil Catherine refused to reveal the true details of Martin's death, but after being told that his mother, Moira (who was diagnosed with MS), was pleading for answers, she revealed the truth. After they went to her home following their date, Catherine spotted Martin at her computer, after which Martin informed Catherine that he found out about her identity as Boz and he had been on the case searching for her. Feeling betrayed, Catherine delivered a hard slap to Martin, who screamed loudly in pain before collapsing and dying. Believing that she had killed Martin, and after a private ambulance picked him up, Catherine hacked into the records, turning Martin Longthorn into Christopher Smith.
The villainess also removed an immense amount of CCTV footage to hide the fact that Martin was picked up at her home, and it was also revealed that during Martin's quest, he baited her with a list of undercover police officers. The officers' lives would have been endangered if she made it public, but even so, the callous Catherine stated that it was a war against corruption. Sandra responded by informing Catherine that her violent actions to Martin weren't because she was exposed, but she felt (in her mind) betrayed by Martin, as she believed she finally found a connection the real world. Catherine's only response came in the form of questioning Sandra's abilities, and she was arrested (off-screen) for her various crimes.