Adric

Adric had a genius for, and badge in, mathematical excellence. He joined the Doctor by stowing away in the Tardis. Socially awkward, he gave his life to save others. He was the 3rd of the Doctor's companions to die while traveling with him, and the first since since The Daleks' Master Plan.

Profile
Adric was orginally a member of a gang of teenage thieves on the planet Alzarius in E-Space with his brother Varsh. He had the ability to calculate vast sums in his head. (DW: Full Circle) Adric sneaked aboard the TARDIS, as the Doctor and Romana would discover later. Cunningly, he pretended to share the agenda of the Three Who Rule on the next planet that they visited. (DW: State of Decay)

After Romana departed (DW: Warriors' Gate) he acted briefly as apprentice to the sage-like Doctor. (DW: The Keeper of Traken, Logopolis) this would change, though Tegan and Nyssa joined the Doctor and Adric and when the Doctor regenerated. (DW: Logopolis, Castrovalva)

Often frustrated by his clumsiness, and fiercely competitive, Adric struggled to find a useful role with the others.

Adric got to the decision of deciding to try to return to E-Space, when, caught up by events, he gave up his life by remaining on board Captain Briggs' freighter, which, travelling through time, crashed into prehistoric Earth and caused the death of the dinosaurs. (DW: Earthshock)

at the end of that episode, the credits rolled in silence, and with his broken mathematical excellence star as the background.