Charon was the ferryman of the Underworld. He ferried the dead across the river, provided they paid him.
When Katarina and the First Doctor approached Charon, they had no money. The Doctor pleaded for Charon to ferry her across the river, but Charon refused. The Doctor offered his signet ring as payment, but Charon wouldn't take Katarina because she was "unclean". The Doctor took Charon's gold from him and scattered the coins. While Charon scrambled around trying to retrieve his gold, Katarina and the Doctor took his boat. He tried to stop them but failed. (PROSE: Katarina in the Underworld)