Jebelex the Pulthasian was a thief.

The Zaan hired him to steal Honey Vox's Treed Crystal. He used a matter transmuter to turn one wall of the safe it was in to liquid to do so. However, when the Zaan turned up to get the crystal, Jebelex had changed his mind. He decided to try selling the crystal to the highest bidder, so fled from the Zaan, the Tenth Doctor and Martha Jones. (COMIC: Jewel of the Vile)

Following his DNA trail, the Doctor and Martha went to the planet Karaten. They found that Jebelex had been eaten and his skeleton and the crystal were inside a liquid creature. Martha used the matter transmuter to freeze the creature, and the Doctor shattered it and retrieved the crystal. (COMIC: Lock, Stocks and Barrel)