Adric

Adric was a native of the planet Alzarius in E-Space. A mathematical whiz (he won a gold star as a prize in a math competition), Adric met the Fourth Doctor and Romana when they visited Alzarius during Mistfall. Adric stowed away on the TARDIS, and the Doctor reluctantly took him on as one of his companions. Adric's mathematical ability aided the travellers on occasion, and he remained with the TARDIS crew when it returned to N-Space.

After the Doctor's regeneration into his fifth incarnation, Adric was used by the Master in an attempt to trap the Doctor within Castrovalva. He was freed, and stayed on a short time thereafter, finally sacrificing his life to stop a plot by the Cybermen.