Liz Picket is the hidden villainess from "Critical," episode 2.07 of Person of Interest.
She was played by Geneva Carr.
Liz Picket is a nurse who was assisting Dr. Madeline "Maddy" Enright in the operation of energy CEO Oliver Veldt. However, Liz is revealed to be a plant working for the episode's main villain, Alistair Wesley, who was blackmailing Maddy to kill Veldt, threatening to kill Maddy's wife, Amy, if she didn't comply.
Liz's heel turn came during the operation, as Maddy discovered that Veldt's artery was nicked by Liz intentionally. Liz stated to Maddy that she was the back-up plan to kill Veldt in case Maddy didn't comply, and it was also revealed that she leaked the info about Veldt's operation to Wesley. After the revelation, the evil Liz told Maddy to let Veldt die, and reached for a scalpel with intent to attack Maddy. Finch, who was planted as a doctor on Maddy's side, knocked out the villainess with an overhead light.