|The Beast of Babylon
Time of Origin: Karkinos; most likely in the far future
"The Beast of Babylon"
History: Although she only travelled briefly with The Doctor, Ali is a companion that deserves to be acknowledged, as she inspired The Doctor to return to the woman who would have a particularly significant impact on his ninth and tenth incarnations.
A Karkinian, Ali's natural appearance was an anthropod with claws and antenodes, possessing four legs and two arms - one arm with a large claw - along with four eyes and a round, sharp-toothed mouth. Female Karkinians had evolved to be fighters to protect their eggs from predators, retaining this knack for combat and violent response even after evolving into a more civilised state by Ali's time.
Educated to at least college level, Ali was aware that TARDISes were theoretically possible and that Time Lords were extinct. Although she considered the technology in the TARDIS 'ridiculously retro', she was proficient in using it, working out how to operate the TARDIS monitor fairly quickly. Her planet was located in a terminus galaxy, resulting in it being a starting-point for journeys to various other races, resulting in her witnessing various travellers while never having travelled anywhere herself.
Ali first met The Doctor at a family picnic when a Starman - an entity formed from a collapsing star, manifesting in the form of giant twins - attacked a group of people nearby. The Doctor was able to drive the Starman off at the time, but Ali pocketed a silver orb which was left behind by The Doctor after the Starman left the area. A few days later, while attending college, Ali met The Doctor again and accompanied him as he hunted a second Starman to Babylon in Earth's ancient history, trading the orb - really a highly-advanced weapon that The Doctor had stolen from its creator to stop the first Starman - in exchange for a trip in the TARDIS.
Although The Doctor initially asked Ali to remain inside the TARDIS while he searched for the Starman, things started to go wrong when Ali instinctively attacked a guard who tried to enter the TARDIS. Although she merely stunned the first guard, when other guards attempted to move the TARDIS and execute The Doctor in the belief that he was a sorcerer who had summoned the Starman, Ali shifted to move lethal methods after a guard tried to stab her shell.
Although Ali's actions allowed The Doctor to recover the orb and use it to drive the Starman back once again, she went on to kill the current king of Babylon and his high priest for their attempts to kill The Doctor. With this in mind, The Doctor subsequently took her back home to Karkinos, feeling that Ali's powerful fighting instinct was fundamentally incompatible with his preference for a more peaceful approach, even if he appreciated that her actions had been committed in his defence. After the TARDIS had returned her to her rightful place on Karkinos, Ali persuaded The Doctor to go back to Rose Tyler, a human he had met shortly before he first met Ali ("Rose"), reasoning that Rose's initial rejection of him in favour of her life on Earth didn't mean that she couldn't be as irresponsible and wild as the next girl.
While Ali only travelled briefly with The Doctor, he made it clear that he appreciated her during her brief time with him, assuring her as he left that she was a perfect example of why he liked to have companions who could inspire him to be more (It was implied that he would return to Karkinos and meet Ali again in the future, but these events have not been depicted yet).