Who is Balthesar from Lamb: The Gospel According to Biff and what is their importance?

Balthesar is the first Magus that Joshua and Biff visit. Balthesar lives for 260 years by keeping a demon named Catch contained within his fortress. Balthesar enjoys the attentions of eight Chinese concubines and lives in luxury. Biff stops all this when he releases the demon while Balthesar and Joshua travel to a temple. They return to find Biff and the only surviving concubine, Joy, fighting the demon. Joshua sends the demon away, but this causes Balthesar to die.