Katarina

Katarina was a companion to the Doctor for a very short time, immediately following the departure of Vicki. She was a Trojan servant who effectively worshiped the Doctor as the god Zeus until her untimely death. She was the first companion to be killed while travelling with the Doctor.

Biography
The Doctor first met Katarina as a little girl living near Troy. She explained to him her ambition of serving as the handmaiden to a priestess, before 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). (DWM: An Unfulfilled Dream)

As a young woman, Katarina served Cassandra, Princess of Troy, and was little more than a slave. Cassandra sent Katarina to spy on the Doctor and his friends, and in particular Vicki, known to Cassandra by the name of Cressida, in order to gather evidence that she was really a spy for the Greeks. To this end, she befriended Vicki, who sent her to help the Doctor get Steven Taylor back to the Doctor's 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. She was from an era which believed in prophecies, auguries, heroic myths, and in gods. She never 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 in order to blackmail the Doctor in returning Mavic Chen's ship, the Spar back 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. She was certain that her death had been foretold, and that she was doomed to die. (DW: The Daleks' Master Plan)

Aftermath
When, after the Doctor had defeated the Daleks, Steve 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 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 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 pProducer 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.
 * She was also the shortest lived companion of the classic era, appearing in five episodes, spread over two stories. Incidentally, her successor, Sara Kingdom appeared in nine episodes, despite being in only one story.