Cassandra O'Brien.Δ17

Lady Cassandra O'Brien.Δ17 (pronounced "dot delta seventeen"), or simply Lady Cassandra, was a former human in the far future. Along with other rich and powerful people of the universe, Cassandra was on Platform One, a space station orbiting Earth in the year 5,000,000,000, set to witness the destruction of the planet by the expansion of the Sun. (TV: The End of the World)

Early life
According to Cassandra, her father, a Texan, and her mother, who came from the Arctic Desert, were the last to be buried in Earth's soil. She was born on Earth and lived on the edge of the "Los Angeles Crevasse". (TV: The End of the World)

Cassandra hinted to Rose Tyler that she was born male, stating "...when I was a little boy, down there..." (TV: The End of the World)

At a party, her future self, using a psychograft to inhabit the dying body of Chip, was the last person to call her beautiful before dying at her side. This memory would stay with Cassandra the rest of her life, as Chip was the last person to admire her beauty. (TV: New Earth)

She was married at least five times. Her life was extended through a series of seven hundred and eight plastic surgery operations until she was nothing but a piece of skin stretched onto a frame with eyes and a mouth, connected to a brain in a jar below. The skin had to be constantly moisturised to keep it from drying out.

As the rest of the human race had long since left Earth and had interbred with other species, Cassandra considered herself the last "pure" human and the others mongrels. Rose characterised her as a "bitchy trampoline" and called her "Michael Jackson". She was guarded by attendants, who sprayed her regularly with a patented moisturising formula kept in canisters. They also served as soldiers when they filled the canisters with acid and killed those who tried to harm Cassandra. (TV: The End of the World) She also "force-grew" the clone Chip, who became her devoted assistant. (TV: New Earth)

On Platform One
Cassandra was a guest on Platform One to witness the destruction of Earth by its expanding Sun in the year 5,000,000,000. Cassandra used metal spider robots to sabotage Platform One's computer systems and the android Adherents of the Repeated Meme to direct attention away from herself as the spiders sabotaged the platform's sun filter. Her original intent was to create the appearance of a hostage situation and collect compensation (with it not being apparent she was the hostage taker) to fund further surgical procedures.

When that plan was exposed by the Ninth Doctor, she teleported off the station, leaving the space station to be consumed — eliminating both the evidence and allowing the shares she held in the guests' rival companies to triple.

However, the Doctor reset Platform One's systems, saving the station, and also reversed the teleportation feed, bringing Cassandra back. In the heat, and without her assistants to moisturise her, Cassandra's skin dried, stretched and exploded, apparently killing her, although her brain was not destroyed. (TV: The End of the World)

On New Earth
Twenty-three years later, Cassandra was living on New Earth. Her brain had survived, her eyes had been retrieved "from the bin" and she was "repaired" with extra skin taken from the back of her previous body. She hid in the basement of the hospital run by the Sisters of Plenitude and was tended to by Chip.

When she discovered Rose and the Tenth Doctor were on New Earth, she lured Rose to her hiding spot in the basement of the hospital and used a psychograft to transfer her consciousness into Rose, taking control of her body.

This was in part to gain revenge on Rose and also to further her lifespan, intending to use Rose's body to live on for centuries. Her impersonation of Rose was marred by her Sloane accent, a poor imitation of rhyming slang, and anachronistic technical knowledge. In the end, however, it was her lack of compassion that gave her away. Her first instinct upon learning of the Sisters' "plague farm" was to blackmail them; when this failed, she released the clones, who in turn, released more of them.

Despite this, while escaping the clones, Cassandra was forced to possess one of them. When she returned to Rose's body she appeared to display remorse and sympathy at the fact the clones had never been touched, after feeling their pain for herself.

Using the lift's disinfectant, Cassandra helped the Doctor cure the clones with the Tenth Doctor, who jubilantly told her that 'life will out', and that these new humans were still living things, no matter how much disdain she harboured for their mixed blood. In the aftermath of the averted crisis, while the Doctor was trying to persuade Cassandra to leave Rose's body, Chip reappeared. Seeing him as a volunteer (which was confirmed by Chip himself) Cassandra transfered her consciousness to him, but the stress of the would-be disaster and his near brush with death earlier, compounded with his relative weakness (being a clone) began to tax Chip. As Cassandra informed the Doctor and the now-restored Rose, "He's dying."

As a final request, the Doctor took her to the party where she had been told she was beautiful. In Chip's body, Cassandra told her past self she was beautiful, before dying in her past self's arms. (TV: New Earth)

Personality
Cassandra was very elitist and looked down her nose at the working-class Rose Tyler, who she considered a "chav". (TV: New Earth) She showed extreme arrogance at times, claiming to be the last "pure human" and considering the new evolved humans "mongrels" due to their intermingling with the natives of the planets they colonised. On Platform One, she was also shown to be selfish, thick-skinned, and devious - willing to sacrifice an entire station of people just for profit. (TV: The End of the World) Her apathetic nature and dislike of "sub-humans" is what ultimately gave her away when she was inhabiting Rose's body, since the Doctor realised that while Rose would care about the new humans' pain, she did not.

Despite her generally callous attitude, Cassandra was occasionally compassionate. After briefly possessing one of the infected new humans, Cassandra displayed genuine distress over their never experiencing touch and all of the pain they were going through. Albeit reluctantly and mainly due to circumstance, Cassandra helped the Doctor save all of the infected people in the hospital and could be seen smiling in happiness until the Doctor told Cassandra that she had helped make the new humans and could not deny them.

Cassandra's fatal flaw was her extreme reluctance to accept death. On Platform One, she rebuffed the Doctor's statement that "Everything has its time and everything dies.", claiming she was too "young". This led her to become callous and highly unapproachable - she watched other people wither and die around her while she grappled onto life with both hands. (TV: The End of the World) The Tenth Doctor was appalled by her willingness to possess other people's bodies just to prolong her lifespan, since it robbed the host of their lives. Only after she realised that Chip's body was dying did she finally accept it was time to die. (TV: New Earth)

As a younger woman, while she still had a full body, Cassandra was far more compassionate, trying desperately to help the strange man who had collapsed after telling her how beautiful she was. This indicates that Cassandra's long life and desperate struggle for survival changed her greatly from the person she once was (TV: New Earth) by making her more indifferent to life in general and to other people in particular. She objectified life and therefore lost sight of the value of it until she was about to lose her own.

Behind the scenes

 * According to the reference book Doctor Who: Monsters and Villains, which is not a valid source for in-universe articles, Cassandra was born as Brian Edward Cobbs, in the ruins of the Walsall Apology, in the old Calendar Year of 4.99/4763/A/15.

 Cassandra O'Brien.Δ17 Cassandra O'Brien  Cassandra O'Brien.Δ17 Кассандра О'Брайен.Δ17