Katarina

Katarina was a companion to the First Doctor for a very short time, immediately following the departure of Vicki Pallister. She was a Trojan servant who worshiped the Doctor as the god Zeus until her untimely death sacrifising herself to save her "god's" life. She was the first companion to be killed while travelling with the Doctor.

Biography
The Seventh Doctor first met Katarina as a little girl living near Troy. She spoke to him of her ambition to serve as handmaid to a priestess, because her poor family could not afford to feed her any longer. The Doctor gave them a gold coin to alleviate their poverty but, upon learning Katarina's name, he realised that she would die young (as he had already witnessed it in his personal timeline). (BE: An Unfulfilled Dream)

As a young woman, Katarina served Cassandra, princess of Troy, and was little more than a slave. Cassandra sent her to spy on the First Doctor and his friends, and in particular Vicki, known to Cassandra by the name of Cressida, to gather evidence that she was a Greek spy. To this end, she befriended Vicki, who sent her to help the Doctor get Steven Taylor back to the TARDIS after a spear thrust had critically injured him. Once there, Katarina tended to Steven's wounds, and joined Steven and the Doctor on their travels. (DW: The Myth Makers)

Katarina came from a simpler time, and never truly understood where she was, or what was happening to her. She mistook the TARDIS for the Palace of Perfection and the Doctor for the god Zeus, come to take her on a journey beyond, in his temple, to the real Palace of Perfection. Despite the Doctor's protestations to the contrary, she remained set in this belief.

When the criminal Kirksen held Katarina hostage in an airlock to blackmail the Doctor into returning Mavic Chen's ship, the Spar, to the planet Kembel and the Daleks, she used what knowledge she had to activate the airlock door and expel herself and Kirksen into space, causing their heads to explode in the vacuum. She was certain that her death had been foretold, and that she was doomed to die. (DW: The Daleks' Master Plan)

Aftermath
After the Doctor had defeated the Daleks, Steven Taylor said that he had seen too much death lately; the sacrifice of Katarina doubtless weighed on his mind, among others. (DW: The Daleks' Master Plan, The Massacre) In the afterlife, Katarina found herself without a coin and unable to cross the River Styx. Due to the intervention of the Doctor (or her idea of him) she found her way not only to Asphodel, where all souls who had done neither good nor evil would go, but to the Elysian Fields, the abode of the blessed. (ST: Katarina in the Underworld)

Much later, on the ruined world of Adeki, the Seventh Doctor found one of several Gwanzulums, beings who used their shapeshifting powers to pass themselves off as some of the Doctor's past companions, including Katarina. Before he discovered the ruse, the false Katarina attempted to have him take her off the planet. (DWM: Planet of the Dead) Later, inside the inner landscape of the Doctor's mind in a Hell haunted by his guilt, Ace came across the ghost of Katarina in the form of an eternally frozen girl of ice. (NA: Timewyrm: Revelation)

Behind the scenes

 * Doctor Who Producer John Wiles and his production team decided to write out Katarina as soon as possible, in order to get around the writing challenges of a character so very unused to modern concepts. Therefore, they decided to do so in the most dramatic way possible.