Christa Pullman is the true main villainess of the 4th season episode "Art Imitating Art" of Elementary.
She was portrayed by Christina Cox, who also played Zoey Kruger in a 4th season episode of Dexter.
Christa Pullman is the villainous district attorney who helped put away Lewis Bowman for the murder of Marissa Kagan. However, it was revealed that Christa was cheating on her wife, Terri, with Marissa, and it was actually Terri who killed Marissa when she found out. Once Christa learned what Terri did, she covered up the crime and hired Zoe Mercado to work on Marissa's body, knowing that Zoe is a "dry labber"; someone who fudges reports to make sure the police arrest who they believe committed the murder. Later on, Phoebe Elliott, showed Christa a selfie that she took which showed Bowman in the picture; which proves Bowman's innocence. Knowing this, Christa doctored the photo; doing so by removing Bowman from the selfie. After that, the evil Christa drove up to a nearby gym and shot Phoebe to death, continuring to cover for Terri, who later attempted to take the fall for Phoebe's murder.