Fourth Drax

Drax was a renegade Time Lord with a knack for tinkering. He knew the Doctor before either had left their home planet of Gallifrey.

Youth
Drax attended Prydon Academy with the First Doctor in the Class of 92. (DW: The Armageddon Factor)

Drax and the Doctor belonged to the Deca, a social group of rebellious students at Prydon Academy. Drax and Mortimus became the first two members of the Deca to permanently flee Gallifrey and turn renegade. (PDA: Divided Loyalties)

As a Renegade
Upon going renegade, Drax travelled the galaxy doing repair and maintenance work in the fields of cybernetics, guidance systems, and armaments.

When Drax's TARDIS suffered a breakdown in its hyperbolics, Drax was caught engaging in criminal activity to procure replacements and did jail time in Brixton on Earth. During the Fourth 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 Third planet, ruled by the Shadow.

The Shadow imprisoned Drax and made him work for him. He agreed to help the Doctor. With Drax's assistance, the Doctor recovered the final segment of the Key to Time. Afterward, Drax stayed on Atrios to help rebuild. (DW: The Armageddon Factor)

Personality
Drax showed extreme technological aptitude. As a youth on Gallifrey, Drax had a skimmer he had built himself. (PDA: Divided Loyalties) However, Drax found himself unable to grasp temporal theory, and he failed the class. (DW: The Armageddon Factor)

During his jail stay Drax picked up Brixton's local accent and idioms, which remained part of his speech patterns even after he'd left Earth and resumed his travels.

When Drax met K9 Mark II and marvelled at the "little tin dog," K9 made note of Drax's silliness.

Although Drax was not above criminal activity and weapons dealing for personal gain, he displayed a sense of what was morally right and bemoaned having to assist the Shadow in his schemes. The Doctor was able to talk him round to joining him against the Shadow without much trouble. (DW: The Armageddon Factor)

Behind the scenes

 * The exchange between the Doctor and Drax regarding his accent in The Armageddon Factor would be echoed decades later in a similar exchange between Rose Tyler and the Ninth Doctor in Rose.


 * The Make Your Own Adventure with Doctor Who book Search for the Doctor included Drax as a character that the reader/main character could interact with. He and K9 Mark III helped the reader defeat Omega and rescue the Sixth Doctor. In this book his TARDIS was in the shape of a convertible car, the license plate of which read "TARDIS."