Baltazar was a space pirate who had once been imprisoned on the prison planet Volag-Noc.
Baltazar was determined to compress the planet Earth into diamond, which would kill all life on the planet. His plan was thwarted by the Tenth Doctor and Martha Jones. Later, Baltazar searched for the lost ship, the Infinite via its data chips. He used the Tenth Doctor to track it down. He had a metal bird pet called Caw.
Baltazar was called "Corsair — King of Triton".
He was famous for having built his own warship, which he considered the finest ever built, by hand over "countless decades," an accomplishment the Doctor considered "really cool", moments before he destroyed it with a spoon.
The Doctor called him the "Scourge of the Galaxy". Baltazar was notable for having a sense of humour — rare among the Doctor's opponents. He poked fun at Martha and laughed while watching the Doctor awkwardly trying to pilot Caw's offspring, Squawk. (TV: The Infinite Quest)