Liz Picket (Geneva Carr) is the hidden villainess from Person of Interest episode 2.07, "Critical." She is a nurse who was assisting Dr. Madeline "Maddy" Enright (the episode's protagonist) in the operation of energy CEO Oliver Veldt. Liz turned heel when she was 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 villainous 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.