Yama is the gatekeeper to the gates of hell. He appears once someone has died to take their souls before they are properly buried, he doesn't care whether the souls are good or bad. Yama belongs to the spiritual realm however he is able to be frozen by Piper's power which helps Mark get away the first time Yama appears. Yama takes the soul of Tony Wong after Mark pushes him onto his lance. He then goes after Mark but the sister's stop Yama from taking him. Yama takes his souls with a large lance that he impales his victims with.
Yama only appears in this episode and only appears breifly when he does.