Karen is a sweet friend of Sophie and Nancy's who Mark meets in the series 2 episode, Wedding (Series 2). She is played by Sarah Hadland.
Karen is not seen together with Nancy with the exception being the day Nancy weds Jeremy.
At a local coffee shop, Gino's, Sophie attempts to set Karen up with Mark during a period where they try to remain friends, but Mark manages to bungle up the first meeting. Still, Karen still thinks Mark is a funny guy when she sees him later at Jeremy's wedding reception.
At the reception (and in a rare instance of bravery) Mark manages to ask Karen out, but again convolutes matters by inviting Sophie and her new interest Sam, along.
It is unclear if she ever goes on her date with Mark.