Adric

Companion of the Fourth and Fifth Doctors. He was a young Alzarian who had earned a badge for mathematical excellence. The Fourth Doctor met Adric on Alzarius in E-Space, where he helped him and Romana solve the mystery of the Marshmen. Adric then stowed away on the TARDIS and helped the Fourth Doctor defeat the Great Vampire, and free the Tharils from Rorvik. Back in normal space, he helped the Fourth Doctor fight the Master on Traken, Logopolis and Earth.  There, he watched the Fourth Doctor regenerate. Soon afterwards he was kidnapped by the Master, who used his mathematical abilities to create Castrovalva. He then fought alongside the Fifth Doctor, Nyssa and Tegan against Monarch, the Mara and the Terileptils and George Cranleigh. After fighting the Cybermen, he sacrificed himself to save Earth by staying on board a doomed spaceship, taking it back through time 65 million years until it crashed on Earth.