According to Nami's mother he was an American marine who was staying in Japan, he was well muscled and towered over her. he had a thin moustache and slicked backed hair, other than that she could barely remember him.
He was present when Nami was still a very young girl. She remembered him, herself and her mother on a beach one time and while she made sand castles, he and her mother played in the water. She could vaguely remember seeing him towering above her, holding her like a little bird in his mighty arms.
However he eventually left with a young blonde girl never to be seen again by the two, because of this her mother became incredibly protective of Nami, sacrificing her beauty and mental health to do so.