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)

It's possible that Lady Cassandra was born male (quote from The End of the World: "...when I was a little boy, down there...") or with no determinate gender. However, she does identify as female, hence the 'Lady' and her female name and pronouns. Or... she could've been joking?

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)

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 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 harbored for their mixed blood. In the aftermath of the averted crisis, while the Doctor is trying to persuade Cassandra to leave Rose's body, Chip reappears. Seeing him as a volunteer (which is confirmed by Chip himself) Cassandra transfers 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) begins to tax Chip. As Cassandra informs 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, who was probably an aristocrat, was very elitist 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 the new evolved humans "mongrels? due to their intermingling with the natives of the planets they colonized. On Platform One, she was also shown to be selfish, thick-skinned, and devious - willing to sacrifice an entire station of people for profit. (TV: The End of the World, New Earth)

Cassandra's fatal flaw is her reluctance to accept death. On Platform One, she rebuffs the Doctor's statement of "Everything has its end and everything dies." This lead her to develop a certain callousness and untouchability - she's watched other people wither and die around her while she grapples onto life with both hands.

In this she represents an extreme of a trait that later enters Tenth Doctor's personality: they both cling to life with iron grips, unwilling or unable to let go. This survival instinct features in the Tenth Doctor's sympathy for life and his reluctance to die. Even as a human John Smith the Tenth Doctor refuses to die (TV: Human Nature/Family of Blood), and even when he's falling over the edge he scrabbles and grabs, desperate to hold on (TV: The End of Time Parts One and Two). He even says, "I don't want to go." In this way, Cassandra is like a little foreshadowing for what Ten will become near the end of his incarnation.

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.

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