Father John Walker is a Catholic priest who has won the respect of the citizens of Drassen. His manners may be slightly unseemly for a member of the clergy - he obviously has a soft spot for Ira and he likes to drink, but he is the one man who is able to send food supplies to the starving rebels hiding in Omerta. With Ira in their party (or another mercenary with a very high leadership skill), this shouldn't be a problem. He is a valuable ally and will boost support for you in Drassen significantly, but he won't join the mercenaries.