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 peoples of the universe, Cassandra was on Platform One, a space station orbiting Earth five billion years in the future, set to witness the destruction of the planet by the expansion of the Sun. (DW: The End of the World)

Early life
Cassandra was born a male on Earth and lived on the edge of the "Los Angeles Crevasse". (DW: The End of the World) At some point, she was biologically altered into a female through advanced genetic engineering.

According to her, Cassandra's father was a Texan and her mother was from the Arctic Desert. She also claimed her parents were the last to be buried in Earth's soil. (DW: The End of the World)

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 moisturized 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 "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. (DW: 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 "force-grow" him. (DW: New Earth) He became her devoted assistant.

Over 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 a hostile takeover of the other guests' financial holdings.

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. (DW: 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 used a psychograft to transfer her consciousness into Rose, leaving her original brain to die. This was in part to gain revenge on Rose and in part to discover what the Sisters were hiding. 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 helped the Doctor put an end to it, albeit largely out of self-preservation. (DW: New Earth)

Death
At the end, Cassandra transferred her consciousness into Chip, who allowed her in willingly. However, Chip's clone body began to fail, and Cassandra accepted that it was time for her to die. Before she did so, the Doctor took her back in time to see herself when she was beautiful and still had a body. In Chip's body, she told her past self that she was beautiful, and collapsed, apparently dying at last in the younger Cassandra's arms. (DW: New Earth)

Behind the scenes

 * Cassanda appears to be very Elitist/Classist and "looks down her nose" at the Working-class Rose Tyler, who she thinks to be "common". Cassandra appears to be of middle/upper class.
 * According to the non-narrative source REF: Doctor Who: Monsters and Villains, 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