Hannah Gregson

Hannah Gregson is the hidden main villainess from Elementary 's sixth season finale, "Whatever Happens, However Improbable."

She was played by Liza J. Bennett.

Introduction
Hannah Gregson is a police officer and the daughter of Captain Thomas Gregson. She first appeared in episode 3.05, "Rip Off," where she was dealing with problems with her partner, Officer Stotz. In her return in episode 3.20, "A Stitch in Time," she asked Joan Watson for help regarding pharmacy robberies, but later showed her ambitious side by apprehending the robbers so she could take credit. However, Hannah regretted her actions and told her father everything.

Hannah returned in episode 6.07, "Give Me the Finger," and it was revealed by Gregson that she was a recovering alcoholic. She was also living with a young woman named Maisie Williams, who was her best friend. Later in the episode, Hannah was informed by Maisie that a man hit her car and wanted to see Hannah. The man was revealed as Michael Rowan, a serial killer, and it was at episode's end that Michael returned to the house and killed Maisie. Hannah found her friend's body during the scene, and she was being comforted in the following episode, "Sober Companions." She identified Michael as the person who hit her car, and she later blamed herself for Maisie's murder.

Episode 6.21
Michael was found murdered in the final scenes of the season's penultimate episode, "Fit to Be Tied," and in the aforementioned following episode, "Whatever Happens, However Improbable," Joan was accused of killing Michael, as Michael made an attempt to kill Joan, and Joan's name was his last word before his death. FBI Special Agent Kim Mallick had been on the case during the last two episodes of the season, and she had been aggressively pursuing Joan under the belief that she killed Michael. Because of this, both Joan and Sherlock Holmes were led to believe that Mallick turned heel in her quest to close the case; not only by killing Michael but setting up Joan in the murder. Their investigation led them to a sanitation department building, where one of the workers revealed that a cop, not an FBI agent, appeared and removed security footage. The cop was revealed as Gregson, and during a confrontation, Sherlock deduced that the killer was Gregson's daughter, Hannah.

Sometime after Michael assaulted Joan, Hannah became a heel and decided to investigate Michael on her own. She tracked him to a friend's home, and it was there that she was dead set on killing Michael. After entering the house, the villainous Hannah took out her baton and beat Michael to death, and after the murder, she enlisted the help of a sanitation worker (whose daughter-in-law was one of Michael's many victims) to help him dispose of the body in one of the trucks. Gregson ended up being an accomplice after Hannah told him everything, and he and even Joan were unwilling to allow Hannah to be imprisoned for her act of revenge, despite Joan being suspected and Sherlock's quest to save Joan from being mistakenly arrested.

Sherlock met with Hannah on the following morning, and it was there that both of them planned to have Sherlock falsely confess, stating that he killed Michael in service of the Queen. This led to Sherlock being ordered back to his native England--which he considered a small sacrifice--with Joan later following him in the final scenes. Hannah's fate, however, wasn't revealed.