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 a male on Earth and lived on the edge of the "Los Angeles Crevasse". (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. The pattern on Chip's body became a favourite of Cassandra's, inspiring her to later grow him. (TV: New Earth)

At some point, she was biologically altered into a female through advanced genetic engineering. She was married several 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 Tyler characterised her as a "bitchy trampoline" and called her "". 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 the ransom and insurance money 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, 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 Doctor jubilantly telling Cassandra she couldn't deny the new humans as she helped create them. The Doctor demanded Cassandra to leave Rose's body. Chip appeared and Cassandra saw a new body to inhabit. Chip openly volunteered Cassandra to inhabit his body. After Cassandra took over Chip's body, she realised his body was dying; though the Doctor offered to help save her, Cassandra finally accepted her death. 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, who was probably an aristocrat, was very and "looked down her nose" at the working-class Rose Tyler, who she considered "common". She showed extreme arrogance at times, claiming to be the last "pure human" and considered all others to be "mongrels" due to their intermingling with other species. She was also very devious, callous and selfish, being perfectly willing to let all residents of Platform 1 be burned to death by the sun just to make money. (TV: The End of the World)

Cassandra's greatest failing was undoubtedly her complete unwillingness to accept death, despite the fact that, according to Rose, it was better to die than live like her. The Tenth Doctor was apalled by her willingness to possess bodies just to prolong her own lifespan, as it robbed the people of their own lives. Only when she discovered that Chip's body was close to death did she finally accept that it was time to die. (TV: New Earth)

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.
 * In The End of the World, Cassandra talks about "the time when she was a little boy", making her the first openly transgender character in Doctor Who.

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