Sonia was Rafael's sister. She was killed by a tidal wave along with Julian and their parents while on a cruise ship piloted by Dartz.
When Rafael survived the tidal wave and the years he spent on a deserted island, he was devastated to return to civilization and learn that his family was gone. After his second Duel with Yami Yugi, Sonia and the rest of Rafael's family appeared to him next to his "Guardians". When The Great Leviathan was destroyed, she was seen again with Julian and their parents, promising Rafael that they would always be in his heart.