3. Jack Boyle
That's right, Jack Boyle (Peter Hermann) is back. The ex-husband of Erin Reagan (Bridget Moynahan) has never been a fan favorite due to stress that he causes Erin and their daughter Nicky (Sami Gayle), but he is a huge part of their lives, and leaving him out of the final season would have been a mistake.
Boyle was last seen in season 13, but he was missing altogether in season 12, and only had one appearance in season 11. His involvement was not made clear until Bridget Moynahan mentioned it during a recent interview with Parade.
The actress also noted that her character has to figure out where she stands with Boyle.