Destrii

Destrii (or Destriianatos) was an enemy-become-companion of the Doctor.

Against the Doctor
The Eighth Doctor and Izzy encountered Destrii inside the gargantuan snake-like spacecraft known as Ophidius, which had swallowed the TARDIS. Destrii has survived on her own inside the belly of Ophidius for quite a while, and was being pursued by the aliens that controlled the world-snake. To evade pursuit, she tricked Izzy into using a machine which placed their minds in each others' bodies. Destrii tried to pose as Izzy and escape with the Doctor in the TARDIS, but the Doctor, observing her violent tendencies, saw through the deception. However, before the Doctor could do anything to reverse the mind switch, Destrii, in Izzy's body, was apparently disintegrated by a member of the predatory (but intelligent) species known as the Mobox. (DWM: Ophidius)

While Izzy was adjusting to the trauma of being trapped in an alien body, she was kidnapped by representatives of Destrii's mother, the Matriax Scalamanthia of the planet Oblivion. Retracing his steps with the help of former companion Fey, the Doctor found Ophidius anchored in the skies above the Mobox homeworld, and Destrii alive in its jungles. It transpired that the Mobox had the ability to retain the patterns of those they distintegrated and reconstitute them. B'rostt, the Mobox who had attacked Destrii, had reconstituted Destrii and left her in the jungles to die, later to build a new empire on his planet. Destrii, however, was made of sterner stuff and had survived, even in Izzy's body. The Doctor took Destrii with him, determined to find Izzy and restore his companion to her rightful body.

On Oblivion, the Doctor discovered that Destrii was a member of a race that had been genetically manipulated so that no two individuals were alike. Generations ago, the people of Oblivion had looked human, until the ruling families attacked their enemies using a plague that stripped its victims' minds bare. The families sealed themselves up to wait out the plague's progress, but discovered that the victims had been transformed instead into living generators of psychic energy who shared a gestalt consciousness. The Horde, as they were now known, transformed the ruling families into animalistic humanoids for their sport and sealed Oblivion from the rest of the universe.

Discovering that Destrii was in Izzy's body, the Horde representatives that kidnapped Izzy switched them back, and Destrii and Izzy saw into each others' minds as the transfer occurred. Izzy saw that Destrii was the daughter of the ruling Matriax, whom the Horde still deferred to. However, Destrii had also been raised in the arena, fighting death duels all her life and was feared by everyone except her uncle Jodafra, who looked like a humanoid lion. Jodafra had built a chronon capsule that Destrii used to escape before, but the Matriax brought her back to be wed. When Destrii refused, the Matriax began to beat her and, pushed beyond tolerance, Destrii killed her mother. The Horde then descended on Destrii, choosing her as their new leader and transforming her into a psychic energy generator as well.

Destrii was about to destroy the cities of Oblivion with her newfound power but Izzy reminded her of her desire to be free and urged her not to become just one of the monolithic Horde. It was then revealed that Jodafra had allowed Destrii to escape to prove his chronon capsule sound, and while the Matriax and the Horde searched for Destrii, he had perfected a device that would drain the Horde to power the capsule for his escape. Realizing that she has been used, Destrii poured her own energy and that of the Horde into the capsule, reverting to her amphibian form and giving her and her uncle the ability to roam time and space. (DWM: Oblivion)

The Doctor next encountered Jodafra and Destrii in North America in 1875 near the Lakota village led by Chief Sitting Bull. Jodafra had made an agreement with the other-worldly spirit known as the Windigo to feed it human flesh in exchange for help navigating the time stream. When Destrii realized that Jodafra meant to sacrifice Lakota children to the Windigo, she destroyed the equipment that supported the Windigo, allowing the Doctor and the Lakota to defeat it. Enraged, Jodafra beat Destrii nearly to death, leaving her battered and unconscious body in the snow and departing in his capsule. The Doctor found Destrii and took her to Hippocrates Base, a space station hospital, to heal. (DWM: Bad Blood)

With the Doctor
There, Destrii aided the Doctor in repelling a Zeronite attack on the base. While the Doctor acknowledged that Destrii was an "egocentric, immature hellion" with a violent streak "wider than the Gobi Desert", he also felt that there was a decent person underneath that surface. He offered her a second chance and to show her the universe if she would stick to the rules, and Destrii gladly accepted. (DWM: Sins of the Fathers)

One of their first trips together was to Earth, where Destrii wore a holographic disguise that made her look human. She accidentally offended a Chinese stallholder. She killed many Cybermen when taken aboard the Cyber-ship with the Doctor. After defeating the Cybermen, the Doctor landed the TARDIS in a field and together they walked off into the distance. (DWM: The Flood)

Personality
Destrii was unique for having a personality unlike many other companions: she took a "fists first, brains second" approach. For a while she was villainous after she stole Izzy's body, but was taken on as a companion after Izzy left. She was on occasion racist but did not mean offence. She was extremely agile, showing remarkable combat abilities and had knowledge of popular Earth culture, but was also initially unprincipled and quick to violence. (DWM: The Flood)

appearance
Destrii was an amphibious humanoid with scaly skin, claws and a face resembling that of a fish. (DWM: Ophidius)

Behind the scenes

 * According to Panini Books' graphic novel reprinting of The Flood, Doctor Who Magazine intended for Destrii to witness the Doctor's regeneration into the Ninth Doctor (in an unpublished alternate ending to The Flood) and travel with the Ninth Doctor in a series of stories set before Rose. The BBC, while they approved the comic strip depicting the regeneration, vetoed any "year 1" stories with the Ninth Doctor. As a result, Destrii's ultimate fate has yet to be chronicled in any official media, and it would not be until 2008 that DWM's comic strip featured an original companion again with Majenta Pryce.