Who is Jape Waltzer from Peace Like a River and what is their importance?

Jape Waltzer is the strange, paranoid conspiracy theorist Davy lives with while on the run. He has procured a young girl, Sara, who he is "raising" to be his wife. Jape murders Mr. Andreeson, and when Davy steals Sara away to protect her from Jape, Jape follows him home and shoots Jeremiah and Reuben.