Adam Mitchell is a self-proclaimed genius from 2012, who met the Doctor and Rose after being recruited by millionaire Henry van Statten to acquire and catalogue alien artefacts for his massive collection. Adam was a prodigy from an early age; when he was only eight years old, he logged onto the US Defence system and nearly caused World War III. He is apparently the only child of Jeff and Sandra Mitchell, and lives with them and their dog in England.
Adam always harbored a desire to travel in space; this, combined with his crush on Rose - and the fact that he doesn't know what he's stepping into - leads him to follow Rose into the TARDIS. He is initially quite overwhelmed by time travel, even to the point of passing out. Once he comes to terms with his situation, his first reaction is greed: with all the information of the future available to him, Adam realizes that he can use Rose's cell phone to pass information back in time to his parents. To facilitate this information transfer, Adam chooses to have an infospike implanted in his head. With a click of his fingers, it unfolds to reveal his brain, and enables a connection to be made that essentially turns Adam into a human computer, processing vast amounts of information and able to transmit it anywhere. Along with the infospike, he also gets a complimentary vomit-o-matic, by means of nanotermites implanted in his throat lining which freeze the waste as it emerges.
Although generally a nice, friendly person with an easygoing personality and a ready smile, Adam's likeability hides a rather adept liar, who is quick to shift blame to others and seems to be motivated largely by self-interest. He would also appear to be a rather poor fighter, although Adam insists that he can defend himself. He is relatively short, only somewhat taller than Rose, with dark brown hair and eyes.
Adam's scheme to become wealthy off the information of the future inadvertently results in him giving the Jagrafess access to everything he knows about the Doctor. When the Doctor discovers that the situation was caused by Adam's greed, and that he had risked the integrity of the timestream by trying to take advantage of future knowledge, he loses his patience and returns Adam home, refusing to allow him to continue travelling in the TARDIS. Because he has future technology implanted in his head, the Doctor advises him to be average and unseen; although as it is activated with a mere snap of the fingers, it is difficult to say how long Adam was able to hide his secret (his mother, at the very least, learned about it within mere seconds of his homecoming).
