Adric, a teenage boy with a gift - and badge - for mathematical excellence, was a member of a street gang on the planet Alzarius, along with his brother, Varsh. Adric, alone and hungry for adventure, stowed away in the TARDIS after Varsh sacrificed his life to save the Doctor and his friends.
Often frustrated by his clumsiness, and fiercely competitive, Adric struggled to find a useful role among the TARDIS crew. He ultimately gave up his life by remaining on board a spaceship that crashed into prehistoric Earth, causing the death of the dinosaurs.
Adric is fifteen, small for his age, wiry and strong, with short, straight black hair. His dominating elder brother, Varsh, was the leader of a juvenile street gang on the planet Alzarius -- and under his tuition Adric learned to lie and steal, activities which are the dark side of his natural optimistic brashness and enormous intellectual curiosity.
Adric never fitted into the gang he was pressed into by his brother, partly because of his superior education, and partly because he is a born non-conformist, even among outlaws. When he met the Doctor his strong sense of self-preservation prompted him to assume an air of subdued innocence and false naivete. Though a disguise, this impersonation reminds us of Adric's very real vulnerability as a mortal (as opposed to the Time Lords). With the last of his family ties broken, Adric stowed away in the TARDIS.
The Doctor wanted to return Adric to Alzarius, but with one thing and another.... Meanwhile, Adric's true character is emerging -- enquiring, intelligent, but definitely and irritatingly a mendacious magpie. The Doctor's view of his responsibility towards the boy shifts; rather than return him to a planet where he will resume life as a criminal, wouldn't a certain amount of education, reform, and expansion of his moral horizon be appropriate...?
--Ganked from his original character description by Chris Bidmead from here
Disclaimer: I am not Adric, nor am I Matthew Waterhouse who portrayed him. Adric is just a fictional character from Doctor Who, companion to the Fourth and Fifth Doctor. He's owned by John Nathan-Turner, the BBC, and no doubt some other people I'm forgetting to mention here.