Detective Abigail Chaffee (Rachel Boston) is a homicide detective who frequently crosses paths with Mary and Marshall's witnesses. Abigail firmly believes you catch more flies with honey than with vinegar, and treats her coworkers accordingly. Though this conflicts with Mary's life view, Mary ultimately respects Abigail for her smart and savvy detective work.
Her willingness to help the WITSEC team isn't purely professional, however, because Abigail and Marshall have a thing for each other.