Who is Markham V. Reynolds Jr. from The Guernsey Literary and Potato Peel Pie Society and what is their importance?

Markham V. Reynolds Jr. is an American publisher who sends flowers to Juliet during her book tour without including a note. After she finally procures his address and writes to thank him, he asks her to dinner. Mark and Juliet begin to date, but when he proposes, she requests time to consider. Mark does not want Juliet to go to Guernsey; he wants her to stay in London and marry him like a reasonable person. When he visits Juliet on Guernsey, Juliet refuses his proposal irrevocably because he does not love Kit.