Who is Martin Barnes and Jewel Crenna from Sweet and Deadly and what is their importance?

Martin Barnes is Catherine's tenant. He rents her property outside of town. Martin is an older man with a wife who is a little on the crazy side. When Catherine reports to the sheriff that she saw Martin driving on the road to the shack the day she found Leona, Martin's wife becomes obsessed with the idea that Martin was having an affair with Leona and then begins to think maybe it is Catherine he was sleeping with. However, when Catherine goes to work the following Monday, she learns from her coworker Jewel, who had a house out near the shack, that she and Martin have been having an affair for a long time. This affair goes to the theme of secrets and causes Catherine to realize how many people in her small town have secrets.