At the age of eight, Rosemary was left by her parents at Russell Square station during one of the earliest web attacks on the London Underground. (PROSE: "UNIT Internal Memorandum") They promised her that they wouldn't be long and would come back soon. (TV: The Bells of Saint John) She was reported missing, but during UNITs clearance of the Underground system they found no sign of her. (PROSE: "UNIT Internal Memorandum") She had in fact been taken by the Great Intelligence, and became its servant. (TV: The Bells of Saint John)
By 2013, Kizlet believed that the Intelligence was her client, and not her master. She became the leader of a group, based in the Shard, that served the Intelligence, and uploaded peoples' souls to the internet.
She had a tablet that "hacked" into humans' minds, and was able to control, among other things, their IQs, their conscience, their obedience and their paranoia. She could also speak through people, as everyone was controlled by the Wi-Fi and she was controlling it.
The Intelligence had told her about the Doctor, and she had prepared for their meeting. When the Eleventh Doctor broke into their facility, he demanded that she release Clara Oswald, to which she refused. Releasing Clara would involve releasing everyone, which she was not prepared to do.
Finally, he convinced her by uploading her to the net. She ordered her employees to let everyone out, and when the Doctor raised their obedience metres to the maximum on her tablet, they obeyed and released everyone, including her.
After the ordeal, the Great Intelligence ordered her to restore all the employees to their "factory settings", effectively erasing everyone's memories. By the time UNIT arrived to arrest her, her memory had been erased and she timidly asked the soldiers where her parents were. (TV: The Bells of Saint John)
While under the control of the Great Intelligence, Kizlet was cold and callous and even considered killing one of her employees. She had a dark sense of humour as shown when she said that they couldn't kill the employee until after he returned from a holiday since she didn't want to be "unreasonable". Kizlet said she loved showing off and showed arrogance which reflected the attitude of the Great Intelligence. She believed that the Intelligence loved humanity in a twisted way and said that it gave its victims "immortality, only fatal" since the Intelligence basically killed its victims but they lived on in the Wi-Fi. Upon realising how it felt to be trapped in the Wi-Fi however, Kizlet panicked and screamed at her employees to release her.
However Kizlet's true personality was actually that of a scared little girl asking for her parents. Having been controlled by the Intelligence since childhood, she didn't remember that she had been a child when she first encountered it. (TV: The Bells of Saint John)