Mercy Hartigan
Mercy Hartigan also known as Miss Hartigan was a human agent of the Cybermen, she assisted them in their attempt to conquer Victorian Britain and ultimately the world.
She was a matron at the St Josephs workhouse (a job for which she visually showed great dislike) but when she met the Cybermen she ended up working for them in exchange for the promise that she would be heralded in the 'New Age'. She had Reverand Fairchild killed in order to gather all the childrens workhouse managers all in one place. When the time was right the Cybermen and their Cybershade minions attacked and massacred all the mourners (save Mr Cole, Mr Scoones, Mr Fetch and Mr Milligan). Miss Hartigan had victorian style earpods inserted into the four survivors and ordered them to bring all their residents to the 'Court of the Cyberking' using the Cybershades as watchdogs.
They led the children to a factory by the docks and made them work the mechanics so that the CyberKing would have enough electrical reserves to rise out of the Thames. Miss Hartigan killed the workhouse owners once they were no longer necessary and had the Cyberleader escort her to the observation deck of the still dormant Cyberking. There she realised the Cybermen had been planning to betray her since she had first allied herself with them. She was then converted into the control unit of the CyberKing which accelerated her brain power massivley but couldnt fully change her personality. She then declared that she was now the leader of the Cybermen before promptly dispatching the Cyber-Leader. Once enough energy was harvested the Cyberking rose up and attacked London with Miss Hartigan as the brain of the ship. However using jackson Lakes TARDIS The Doctor reached her and used a device made from Infostamps to break the link between her and the Cybermen. She then saw what she had become and overcome with remorse and horror overloaded the machinery and destroyed both the Cybermen and herself. (DW: The Next Doctor)
Personality
her personal character is that of a woman so twisted by hate and rage she has become merciless compasionless, but also extremely driven and resentfull of her second class status as a woman in a Victorian era. the extent of her negative emotions and drive is so great even cyber-conditioning failed and her mind was able to take control of the cybermen, once attached to the cyberking. it can be implied from dialogue that she had been victimized or abused sexually at some point in her life (though this is fan speculation) which would explain her negative emotions and specific hatred for men. For example when confronted by the Doctor she brands him as " another man come to ascert himself against her in the night" and during the Reverand Fairchilds funeral she calls Mr Scoones a 'cheeky boy' after accusing him of looking at her on several occasions.
