Lady Cassandra

Lady Cassandra O'Brien.Δ17 (Pronounced "dot delta seventeen") is a former human in the far future who serves as the secondary antagonist of Season 6, of which she first appears in the third episode.

According to Cassandra, her father was a Texan and her mother was from the Arctic Desert. In the distant future, her parents were the last to be buried in the Earth's soil. She had been married at least 5 times.

Her life had been extended through at least 708 plastic surgeries, resulting in her being reduced to a piece of skin stretched onto a frame with 2 eyes and a mouth, connected to a brain in a jar below. The skin needs to be constantly moisturized to keep it from drying out, something the Emaciated Master does in exchange for her loyalty to him.

Personality
Cassandra is very elitist and shows extreme arrogance at times, even to her superior, the Emaciated Master. When she's a guest at a party, she is shown to be selfish, thick-skinned and devious, willing to leave the guests to be killed by Cybermen just for the profit that the Master had promised her.

Despite her generally callous attitude, Cassandra is occasionally compassionate, which leads to her death in the Season 6 finale when the Master leaves her to be fried by the Beast's fire breath. Her fatal flaw is her inability to accept death. When the Master says to her "everything has its time, and everything must die.", Cassandra rebuffs his claim by yelling that she's "too young" before the Beast's fire breath incinerates her.