Megan Gallagher as Catherine Black (seasons 1-2 main; recurring episode 12 of season 3) - Catherine was a clinical social worker who counseled crime victims and confronted challenging cases.
Terry O'Quinn as Peter Watts (season 1-3) a high-ranking member of the Millennium Group that often works with Frank on cases, though their friendship dissolves in season two and he takes on a more antagonistic role in season three.