To observe the way the Doctor picks his companions, you’d think he was a fairly come-one come-all kind of Time Lord. Certainly, until Adam Mitchell came along, no one was aware that his selection process was any more rigorus than that of, say, a bored and lonely trucker picking up hitch-hikers for company.
However, the Ninth Doctor was not like other Doctors. He’d already seen too much, in the Time War, his defences were up, and he’d developed a very military bearing. Anyone wishing to thumb a ride with the Ninth Doctor had better have a really good reason for doing so, and act in a very proper and regimental fashion.
The only person who could get past all this was Rose, to whom he acted like a beloved and indulgent uncle, and as she appeared to have taken a liking to this cocky nerd, he was allowed to come aboard. And just like that beloved and indulgent uncle, he takes great pleasure in undermining pretty-boy Adam – played by Bruno Langley – in front of Rose. Like this:
The sad thing is, the Doctor was right all along. Adam’s a craven meddler. He’s someone who has the capability to understand and interact in the future worlds he now has access to, but he also tries to turn this to his advantage. Having been trained by Van Statten, the collector of alien artifacts who had a real live Dalek, his instinct is to send a ton of information about the future back to himself in the 2000s, so that when this crazy ride comes to a halt, he can make a tidy sum from his knowledge.
Here he is, finding out just how different the future really is:
So, he has mutilated himself in order to gain power and influence, in the middle of what should already be a fantastic, life-changing trip, a gift from his new friends. If there’s any common thread that links all of the companions it’s that they don’t seek to use the Doctor at all. They are of use to the Doctor, as company and often a moral compass, and their reward is to travel and experience alien worlds (which often consist of metal corridors, quarries and a lot of weird things with guns). But none of them uses him as a way to better their situation back at home.
Never mind that he’s a little cowardly, or that his implant puts the Doctor and the TARDIS at risk, plenty of companions have done that; the reason Adam is taken home and barred from future outings is simply that he’s too quick to try and turn this situation to his advantage, without considering the consequences. He failed to realise that just being in the future was advantage enough, and he’s a social climber, and that’s something that none of the Doctors have ever been very patient with.
So, Adam is the companion that proves the worth of all of the other companions. He’s the man who failed a selection test he didn’t even know he was taking. And he’s also the guy who will always be sick in ice-cube form.