He is an ancient Egyptian pharaoh who had sealed his own spirit/soul within the mystical Millennium Pendant. His spirit then took on the identity of Yami Yugi, which resided in the body of Yugi Muto, after Yugi solved the Millennium Puzzle. He is the centerpiece of the Yu-Gi-Oh! series next to Yugi and is the driving force behind most of the events that transpired during the series.