Karen Mills is a paralegal living in Queens, New York.
By posing as a waitress at Wayne Kruger's anniversary party, Karen was able to swap Kruger's anniversary video with one containing footage of his extramarital affair. After Shaw provided him with the picture of a waitress other members of the waiting staff hadn't seen before, Finch identified Karen and had Carter pay her a visit.
After Carter pulled over her car and asked her why she was going after Kruger, Karen revealed she was, at one point, engaged, but that the relationship ended when her co-workers found intimate pictures of her taken by an ex-boyfriend, using Kruger's Lifetrace. She also told Carter of a package she received and how she grabbed the chance for revenge it offered.
Carter let her off with a warning, telling her not to do anything like what she had done again, sympathetic to her plight. She also had Karen deliver the package to Fusco, to help figure out who was targeting Kruger.