As revealed by Captain Thomas Gregson, Marilyn had turned down Gregson's request for a warrant regarding the NYPD's investigation into serial killer Michael Rowan, who strangled numerous women to death. It was also uncovered that there were problems in Marilyn's marriage, as Daniel was having an affair with his co-worker, Rachel Garner--an affair that Marilyn found out about and became enraged over. She then realized that she had full knowledge of Michael's murderous history, as well as his M.O. for killing his victims, and with that in mind, Marilyn turned heel and decided to use that knowledge in a plan to kill Rachel.
The villainess found out that Dan and Rachel would meet on Tuesdays, while Marilyn was at her Bar Association meeting. Rachel would often take a swim in their pool while waiting for Dan, and knowing this, the evil Marilyn appeared at the pool and strangled Rachel to death. After the murder, the maniacal villainess placed the body in plain sight: at a place where Sherlock Holmes went to his meetings. Marilyn's plan worked to fruition; Michael was accused in the murder, but he denied killing Rachel, while telling Sherlock that whoever killed Rachel was angry at her. Meanwhile, Marilyn again denied Gregson's request for surveillance on Michael, mainly to keep him from finding out that Marilyn was the true killer.
However, Sherlock and Gregson's visit to Dan's workplace ended up revealing Marilyn as the villainess, as they returned to her office afterwards and fingered her as Rachel's killer. The pair uncovered Marilyn's scheme to kill Rachel and set up Michael, while revealing the affair as motive, with Marilyn arrogantly responding that there was no proof to back up the claims. After Gregson's plea for Marilyn to think of the families of Michael's victims, Marilyn confessed (off-screen) to the murder, and she was later arrested (also off-screen).
- Tonya Pinkins also played recurring villainess Ethel Peabody on Gotham.