Cassandra O'Brien.Δ17

The ninth incarnation of the Doctor encountered the Lady Cassandra O'Brien.Δ17 (pronounced "dot delta seventeen"), or simply Lady Cassandra, during the events of the destruction of Earth. They, along with the rich and powerful of the universe, were on Platform One, a space station orbiting Earth five billion years in the future, set up to witness the final destruction of the planet by the expansion of the Sun. (DW: The End of the World)

Early life
According to her, Cassandra's parents were the last to be buried "in its soil". She had been born a male on Earth, and lived on the edge of the "Los Angeles Crevasse". At some point, he was biologically altered into a female through advanced genetic engineering. She says that her father was a Texan and her mother was from the Arctic Desert. Married several times, her life had been extended through a series of 708 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 as mongrels. Rose Tyler characterized her as a "bitchy trampoline" and referred to her as "Michael Jackson". (DW: The End of the World)

On Platform One
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. 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 was exposed by the Doctor, she teleported off the station, leaving the others to die — eliminating both the evidence and allowing a hostile takeover of the 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 stretched and exploded, apparently killing her, although her brain was not destroyed. How Chip saved her brain is in question. (DW: The End of the World)

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

When she discovered that Rose and the Tenth Doctor were on New Earth, she used a psychograft to transfer her consciousness over Rose's own, 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 attempt to blackmail them; when this failed, she helped the Doctor put an end to it, albeit largely out of self-preservation.

Death
In 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. This would be remembered by Cassandra as the "last time someone called me beautiful." (DW: New Earth)

Behind the scenes

 * Cassandra told Rose that she used to play on the earth when she was "a little boy," making her the first apparently transgender character on a televised Doctor Who story. However, her subsequent appearance in New Earth makes no reference to this and her younger self, seen at the end of the episode, is depicted as being female. This may have been a joke as in a man saying, "I haven't laughed that hard since I was a little girl." However, in DW: New Earth she seems unfamiliar with the Doctor's male physiology after performing a psychograft into him, implying that she did not change from a man to a woman, since she seems to have no experience with a male body and instead seems to have an obscene fascination with it. Doctor Who: The Ultimate Monster Guide's section on Cassandra features an excerpt from an in-universe book, "Lives of the Rich and Thin", which implies that Cassandra was once a man named Brian Cobbs, who changed his gender in order to marry the head of the company where he worked as a security guard, and later kill him.
 * 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.