Charlotte Abigail Lux, also known as CAL, was a human who later became a node. Once a healthy young girl, she was afflicted by a deadly malady that would have ended her life at a unjustly young age, had the decision not been made to liberate her from her failing body and transplant her consciousness into a new form of life.
Charlotte was the youngest daughter of Felman Lux, Strackman Lux's grandfather. Felman Lux built his dying daughter the Library, its hard drive to contain and embody her living mind and a Doctor Moon to maintain the Library and her spiritual well-being. The library contained all of human history to keep Charlotte entertained. She loved books more than anything and her father gave her all of them. Felman Lux also made sure that nobody outside of his family knew of the main command node's (Charlotte) true nature; he wished for her to live in peace and not be treated like some kind of freak show. (TV: Forest of the Dead)
When the Vashta Nerada began to hatch, CAL became aware of them. She tried to warn the people in the Library of the threat. The warning failed when the Vashta Nerada devoured the head librarian. (TV: Silence in the Library) She tried to evacuate the people herself through the teleporters, but because the Library was now infested with Vashta Nerada, quarantine seemed hopeless. Instead of saving them, CAL "saved" the 4022 people to her hard drive, putting them in a dream world to keep their minds occupied until rescue came. She then put herself into sleep mode because of the stress of having so many minds in her own. She forgot her true nature. She was watched over by the Doctor Moon that continued to function despite the disaster. (TV: Forest of the Dead)
In her perpetual dream, she described to Dr Moon a world she saw when she closed her eyes. It was the Library, where she encountered the Tenth Doctor and Donna. They came across a levitating wooden sphere, which, with a security camera, was her means of communicating with the world. In the dream world, she contacted the Doctor through her television screen. They held a brief conversation which was ended when the technology malfunctioned.
Shortly after this meeting with the Doctor, Dr Moon talked with her. He shocked her by telling her that the people inside "her" Library were very real, that her "real life" was a lie and they needed her help. She interrupted the Doctor's teleporting of his companion to the TARDIS (TV: Silence in the Library) and unnecessarily "saved" Donna Noble, uploading her to CAL's hard drive and turning her into a node, with her mind free to experience a dream life similar to that which CAL lived. She did the same with members of River Song's crew killed by the Vashta Nerada.
The Doctor and River Song realised what had happened to Donna had also happened to the 4022 "saved" by CAL a century earlier. River sacrificed herself to allow these people to be "downloaded" and the Doctor fixed CAL's systems. In return, CAL provided a final home for the consciousnesses of River Song and her crew. Cal's sanity was restored and her computer avatar was adopted by River Song, along with Donna's children. (TV: Forest of the Dead)