what part of Angel would remain with lou Ann
This answer is on the Chapter 2 on the first page in the first paragraph. It says:
"Lou Ann Ruiz lived in Tucson, but thought of herself as jsut an ordinary Kentuckian a long way from home. She had acquired her foreign last name from her husband, Angel. As it turned out, this was the only part of him that would remain with her. He left on Halloween."
His last name is the only thing that remained with Lou Ann.
(The ones in BOLD are also the answer.)