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 then, ultimately, the world. (TV: The Next Doctor)

Alliance with the Cybermen
Miss Hartigan was matron of the St Joseph's Workhouse (a job for which she showed great dislike). Some short time prior to Christmas 1851 she met the Cybermen and agreed to help them achieve their goals in return for the promise that she would be heralded when their plans were accomplished. She was also protected by a guard of Cybershades who drove her horse and coach.

When the Reverend Fairchild was killed, Miss Hartigan went to his funeral and set the Cybermen on the mourners, capturing the workhouse owners and killing the rest. The survivors were fitted with earpieces and told to bring their children to the Cybermen's base of operations.

Rise of the Cyberking
They led the children to a factory by the docks and made them work the machinery to generate electricity so that the CyberKing would rise. Miss Hartigan killed the workhouse owners and the Cyber-Leader told her she would be converted into the CyberKing. She realised they had been planning to betray her the whole time and tried to refuse to be converted but was forced into it.

However, once converted, Miss Hartigan's superior mind gained control over the Cybermen's technology. She not only killed the Cyber-leader but also assumed leadership over the others. Shortly after this, the Cyberking rose out of the Thames and attacked London with Miss Hartigan as the "brain" of the vessel.

Death
The Tenth Doctor then used a device made from Infostamps to break the link between her and the Cybermen. She saw what she had become and, overcome with remorse and horror, destroyed both herself and the Cybermen in a suicidal attack. (TV: The Next Doctor)

Personality
Mercy Hartigan was cruel, callous and sadistic in her interactions with others, especially men, for whom she seemed to harbor a mysoginistic hatred. She possessed a dark sense of humor and took pleasure in watching the mourners get massacred, testing the EarPods on Scoons, Fetch, Cole and Milligan and then killing them afterwards. (TV: The Next Doctor)

Behind the scenes

 * During the podcast for The Next Doctor, Russell T Davies (the writer of the story) confirmed that Miss Hartigan was a victim of sexual abuse and that is why she sexualises every conversation and scenario she is in. This also explains her cruel and sadistic attitude toward men. However Davies felt that especially for a Christmas special this aspect of her would only be lightly touched upon for the safety of a young viewing audience.
 * When Miss Hartigan tests the EarPod's effectiveness, she mimics Mr Crane. Both ask their victims to turn right, then left.
 * Mercy's proclamation of "I am the CyberKing and you will obey me" is a direct parody of the Master's catchphrase, "I am the Master and you will obey me."