Her last memories were of her parents leaving and promising to come back soon. Afterwards, Kizlet was contacted by the Great Intelligence, and became its servant.
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, her body then dying. She ordered her employees to let everyone out. When the Doctor raised their obedience metres to the maximum on her tablet, they obeyed and released everyone, including her.
After speaking to the Great Intelligence and saying goodbye to Miss Kizlet, it orders her to restore their employees to their "Factory Settings" effectively erasing everyone's memories to avoid detection including her own. Kizlet, now having no memory of the last few decades, then asked a UNIT soldier 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)
Behind the scenes Edit
According to The Doctor: His Lives and Times, her first name was Rosemary and she was taken over by the Intelligence at the age of eight after her parents had left her at Russell Square station during the events of The Web of Fear. This was not derived from any information given on-screen.