Who is Martin Tenbones from The Sandman: A Game of You and what is their importance?

Martin Tenbones is a citizen of the land. He is described as a big dog-like creature, who travels to New York City to seek the princess. When he is shot, Martin sees and calls out to Barbie, who recognizes him as her best friend from her dreams. He gives her the Porpentine before he dies and apologizes for letting her down.