Adric

"Now I'll never know if I was right." Matthew Waterhouse, Earthshock

Adric was a companion to the Doctor in his fourth and fifth incarnations.

Biography
Adric and his brother Varsh were members of a gang of teenage thieves on Alzarius, a planet in E-Space. He had the ability to calculate vast sums in his head, who which he won a gold badge for mathematical excellence (DW: Full Circle). Adric stowed himself aboard the TARDIS, as the Doctor and Romana would discover later. Cunningly, he pretended to share the agenda of the Three Who Rule, a trio of vampires who lorded over a small community. (DW: State of Decay)

After Romana left (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)

As the Scorpion King
Following interference by the Doctor during an attempt to recover his TARDIS, his sub-consciousness assisted Adric in breaking the final logic code on the alien computer. Allowing Adric to crash land (though survivable) on Earth. Adric survived for 500 years, creating a City of Excellence and ruling over a sentient race of Scorpions that evolved rather than the dinosaurs. The world they and Adric inhabited was created by Adric from Block Transfer Computations. Adric (after 500 years) re-met up with the fifth incarnation of the Doctor and Nyssa, and accompanied them back to the Victorian era. Adric finally died after assisting the Doctor to recover his TARDIS from Thomas Brewster. (BFA: The Boy That Time Forgot)