When the sixth Doctor was put on trial for meddling in the affairs of other races, the Valeyard was appointed as learned court prosecutor. He was extremely single-minded in his duty, constantly trying to trick the Doctor, undermine his credibility, and attack him from every possible angle. The Doctor retaliated by hurling unflattering nicknames at his opponent: Boatyard, Graveyard, Farmyard, Scrapyard, Knackers' Yard, Brickyard, Backyard, Railyard, and Stackyard.
But there was another layer to the Valeyard. He was not just a man determined to see justice served - he was in the service of the Doctor's archenemy, the Master. And not only that...as the Master revealed, the Valeyard was an amalgamation of the Doctor's evil side, taken from somewhere between his twelfth and final regeneration. If the Doctor was found guilty, the Valeyard was to have inherited his remaining lives.
With his secret out, the Valeyard fled into the Matrix and ensconsed himself within a deadly fantasy world in hopes that he could kill the Doctor and still get his remaining regenerations. His favorite weapon was, apparently, exploding quill pens! But the Valeyard also had another trick up his sleeve - a plan to assassinate the entire High Council with a megabyte modem, aka a particle disseminator. The Valeyard appeared to fall victim to his own scheme, but managed to escape and either take over or replace the Keeper of the Matrix...
In full courtly dress, the Valeyard wore long black robes with a stiff black collar edged in white, and a black scullcap. He is, however, very good at disguise. (What renegade Time Lord worth his salt isn't?)
