Katarina was a companion of the First Doctor.
A Trojan servant, she worshipped the Doctor as the god Zeus and sacrificed herself to save his life. She had barely any contact with other companions, travelling with Steven Taylor on one adventure and very briefly encountering Vicki Pallister before her departure.
Biography
Early life
Katarina first met the Doctor in their seventh incarnation when she was 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, on learning Katarina's name, he realised that she would die young (as he had already witnessed it in his personal timeline). (PROSE: An Unfulfilled Dream, TV: The Daleks' Master Plan)
Meeting the Doctor
As a young woman, Katarina served Cassandra, high priestess of Troy, as little more than a slave. Circa 1200 BC, Cassandra sent her to spy on the First Doctor and his friends, with the particular aim of gathering evidence of Vicki Pallister (known to her as Cressida) being a Greek spy. Katarina befriended Vicki, who sent her to help the Doctor get Steven Taylor back to the TARDIS after a spear-thrust had badly injured him. Katarina tended to Steven's wounds and joined Steven and the Doctor on their travels whilst Vicki remained in Troy. (TV: The Myth Makers)
TARDIS travel
In search of medicine for Steven, they landed on a planet and went through a temporal anomaly. The Doctor, Katarina and Steven visited the Lakhotha tribe whilst the Doctor prepared to go back through the anomaly. Drinks-Pools-Dry showed an interest in her. Later that evening, the tribe and its visitors gathered for a meal. It was enjoyable but everybody (including Katarina) begun to undergo violent transformations into animals and plants. She and Steven did not change as quickly, however. The Doctor and his companions rushed back to the TARDIS, finding the transformations were a result of a being called the Conduit. With Katarina and Steven losing memory of the recent events and Steven's life-threatening injury returning, they promptly left the planet to search for help once more. (PROSE: Scribbles in Chalk)
In the Time Vortex, the TARDIS collided with a future version of itself. This had the effect of preventing Katarina, the Doctor and Steven from landing on Kembel and thus avoided Katarina's death. They spent three months on a planet as a result of damage to the TARDIS, during which she had nightmares about her death and believed that her descent to the Underworld was overdue.
The planet was attacked by the Daleks, whose defeat on Kembel had been averted. Katarina met Jamie McCrimmon and, later, the Second Doctor and Zoe Heriot and had her beliefs about her overdue death confirmed. (AUDIO: Daughter of the Gods)
Death
After this, they landed on Kembel. Katarina stayed with Steven in the TARDIS whilst the Doctor went outside to look for drugs. Bret Vyon gave her some tablets to cure him. As the Daleks surrounded the TARDIS, the trio evacuated into the jungle. After reuniting with the Doctor, Bret, Steven and Katarina headed to the spaceport to steal Mavic Chen's ship, the Spar. As a result of a Dalek tractor beam, the crew crash-landed on Desperus. The Doctor and Katarina worked to prevent any intruders from boarding. Unbeknownst to them, Kirksen had stowed away by the time they had taken off for Earth again. When Katarina checked if a door was secure, Kirksen emerged and held her hostage.
Kirksen escorted her to an airlock to blackmail the Doctor into returning the Spar to Kembel and the Daleks. She deliberately activated the airlock door and expelled herself and Kirksen into space, causing their deaths in the vacuum. (TV: The Daleks' Master Plan)
As she fell through space, struggling to breathe, she thought that gods like sacrifices before finally perishing. (PROSE: Skipping Song) She was certain her death had been foretold and that she was doomed to die. (TV: The Daleks' Master Plan)
Aftermath
After the Doctor defeated the Daleks, Steven said that he had seen too much death lately; the sacrifice of Katarina weighed on his mind, among others. (TV: 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 First 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. (PROSE: 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 realised the ruse, the false Katarina tried to have him take her off the planet. (COMIC: 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. (PROSE: Revelation) The Doctor also once saw Katarina, with Sara Kingdom and Adric amongst his past guilts. (COMIC: The Last Word)
While in the Divergent Universe, the Eighth Doctor was shown an illusion of Katarina by the Kro'ka. He stated that he regretted taking Katarina from her own time and giving her hope, only for her to be killed shortly afterwards. (AUDIO: The Last)
At one point in her life, River Song set out to retrieve Katarina's corpse just after her death so that she could give the young woman a proper burial, feeling that Katarina deserved better than to be left behind after her sacrifice for the First Doctor even if River understood that the Doctor hadn't had a choice but to leave her behind. After River retrieved Katarina's body and left her in a casket, her ship was attacked by the Nine, an insane Time Lord who sought to "collect" all of the Doctor's companions, who had been sent to this time by River's own future self to distract him while River attempted an escape with the aid of the Eighth Doctor's companions Charley, Liv Chenka, Helen Sinclair and Bliss. The Nine was able to retrieve Katarina's corpse and return to his base, but when he attempted to scan Katarina's mind to determine her identity, the residual artron energy in her body gave her dead body enough mental strength to distract the Nine at a crucial moment, allowing the other five women to defeat him. (AUDIO: Companion Piece)
Legacy
The TARDIS, in an attempt to warn the Seventh Doctor about the threat of the Timewyrm, projected one of his memories of Katarina on the scanner. (PROSE: Timewyrm: Genesys)
A photo of Katarina was present in the Black Archive. (TV: The Day of the Doctor)
The Doctor said he will always remember her as one of the daughters of the gods. (TV: The Daleks' Master Plan)
Personality
Katarina came from a simple time and never truly understood where she was or what was happening to her. She mistook the TARDIS for the Place of Perfection and the Doctor for the god Zeus, come to take her on a journey in his temple to the real Place of Perfection. Despite the Doctor's protestations to the contrary, she remained set in this belief. (TV: The Myth Makers, The Daleks' Master Plan)
She followed the Doctor's instructions to the letter and was anxious when she went against the Doctor in the interests of aiding Steven's recovery. She had no knowledge of more technologically advanced ages. As a result, the Doctor educated her on some of these items, for example the purpose of keys. (TV: The Daleks' Master Plan)
She was certain that she was doomed to die, as had been foretold. She deliberately brought about her own demise to save the lives of the First Doctor, Steven Taylor, Bret Vyon, and "perhaps the lives of all the other beings of the Solar System." (TV: "The Traitors")
Behind the scenes
Doctor Who producer John Wiles and his production team decided to write out Katarina as soon as possible to get around the writing challenges of a character so very unused to modern concepts, and to whom the Doctor would have had to explain everything — for example, Katarina did not even know what a key was. They decided to do so in the most dramatic way possible.
Katarina was the first companion of the Doctor to die; she would be followed by Sara Kingdom later in the same serial.
Only one episode featuring Katarina is present in the BBC Archives.