Thirteenth Drax

Drax is a Time Lord who went to the same school as the Doctor and helped him when he was attempting to obtain the Key to Time

During the Doctor’s search for the final segment of the Key to Time, the Doctor and Romana arrive on the planet Atrios, which was at war with their neighbouring planet Zeos. In reality, the planet Zeos is deserted save for a giant computer called Mentalis, which is controlling the war. The true enemy is a third planet called the Planet of Evil, ruled by "The Shadow".

On the Planet of Evil, the Doctor encounters Drax, who is an old Academy friend, and a fellow Time Lord, who had been employed under duress by The Shadow but agrees to help the Doctor. Through his assistance, The Doctor is able to recover the final segment of the Key to Time. (DW: The Armageddon Factor)