Adric

Adric was a young Alzarian who stowed away in the Tardis. He was a genius with mathematics and he became the Doctors companion. Though he had a lack of common sense, he often got the Doctor and himself into trouble. He died heroically, opposing the Cybermen in an attempt to save the Earth from a collision with a freighter full of bombs. It turned out that the freighter was the large mass which wiped out the dinosaurs.

Adric was in Full Circle to Earthshock, plus an illusion in Time-Flight: Episode 2 and a memory in The Caves of Androzani: Episode 4.