Fourth Drax

A renegade Time Lord with a knack for tinkering, Drax had known the Doctor before either left their home planet of Gallifrey.

Youth
Drax attended school on their home planet Gallifrey with the Doctor. (DW: The Armageddon Factor) Both belonged to the social group known as the Deca. (PDA: Divided Loyalties)


 * At some stage, Drax decided to strike off from Gallifrey and travel the universe.

As a renegade
Drax did jail time in Brixton on Earth and some of the local accent rubbed off. When the Doctor's search for the sixth and final segment of the Key to Time, the Doctor and Romana arrived on the planet Atrios, which was at war with their neighbouring planet Zeos. In reality, the planet Zeos was deserted save for a giant computer called Mentalis, which was controlling the war. The true enemy was a third planet called the Planet of Evil, ruled by The Shadow.

The Shadow had imprisoned Drax and made him work for him. He agreed to help the Doctor. With his assistance, The Doctor recovered the final segment of the Key to Time. (DW: The Armageddon Factor)