Leela

The name of an AI in the computer game series, Marathon. Leela was responsible for human-computer interaction on the colony ship and was the first to warn the ship of attack by Pfhor slave ships. Unlike Tycho and Durandal, Leela attempted to help the humans on board the Marathon from enslavement by the alien race.

However, her efforts were in vain as evidenced by the final screen:

"While Tau Ceti was being nuked down to bedrock in 2794, Pfhor scientists disassembled and removed the AI Leela from the Marathon, loading her aboard a vessel bound for the Pfhor homeworld. But the ship fell into the hands of a Nar privateer between jumps at Beta Naxos, and was never seen by the Pfhor again.

Thinking the cargo little more than scrap, the Nar captain sold the Pfhor ship, Leela and all, to a Vylae merchant. The subsequent crash of the Vylae FTL network when Leela was reassembled and reactivated is still legendary in the annals of rampancy, and the Vylae have long since accepted that they will never expunge her from their fifteen-world network. "

Leela of "Doctor Who" was fantastic. Possibly the best sidekick he ever had, because she fulfilled all of the essential reqirements for a Doctor Who sidekick:

Looks

The young Louise Jameson was hot, and all she ever wore in the series was a skimpy hide tunic, a (hide) bra, and (hide) pants. Ohh yeah. If this offends you, remember that this was primetime 1970's TV, and you had to have pretty girls. This garb was, of course, much more suitable for running around evading deadly peril than any dress. (Yes, Sarah Jane worse dresses). Also, the looks of the sidekicks tended to vary, but Louise Jameson represented a high point.

Ignorance and Curiousity

The most essential function of the Doctor Who sidekick was to prompt the Doctor to explain and elucidate. While previous and future (at least the most objectionable ones) sidekicks asked questions because they were plain stupid, Leela asked them intelligently, and she had good reason:

Intelligence

Leela was a paleolithic huntress, so almost anything was new to her if she hadn't encountered it on the series before. The thing was, she was quick on the uptake, so where most sidekicks would respond to an explanation with something like "I don't understand, Doctor", Leela's stock reply would to be more intelligent, either relating it to other things on the series, or asking some question about the thing in question's function. This kind of dialogue hadn't been seen since the Doctor's granddaughter Susan in the time of the first Doctor.

Smarts

The other thing about being a paleolithic huntress is that you get quite good with danger (Trust me, I know these things.) This meant that unlike previous sidekicks, typical twists in the tale would run along the lines of "Doctor steps of camera. Bird touches bad thing. When Doctor comes back, he adds 'saving sidekick's arse' to his overlong list of tasks'. A typical Leela twist would involve her and the Doctor being separated, and one or both of them getting into trouble, and one or both of them escaping, and quite possibly one or both of them getting into even worse trouble.

This girl knew how to run away from things, sneak up behind them, and hold a stone knife to their dirty no-goodnik throats.

Violence

It is important to realise that Tom Baker's Doctor was the apotheosis of the civilised, violence despising Doctor persona. As such, Leela was the one who would be using violence if there would be any.

Why did this happen

Because simpering, useless female characters are boring.

If you must have more depth, Tom Baker really pushed to have an interesting supporting character, and the show had had relatively new, young production staff during this series. Sidekicks such as Leela, and later Romana, were part of an attempt to be really good. The Baker seasons were marked by an attempt to move away from formulaic Sci-Fi (which the show had had a large hand in forming) to really credible screenplays; this was in part a backlash against the hideous Jon Pertwee years, in which ridiculous happenings were made worse by the soldiers of UNIT, whereupon the Doctor would use his superhuman capabilities, advanced scientific knowledge, and karate to solve the problem. The Baker years tended to feature more pure problem solving (in the sense that anyone who knew what was going on could have done it), when that was required, and interaction with the new characters in each story.