He's an ancient Egyption King who went into Yugi's body when he solved the Millenium Puzzle that Yugi's grandpa gave him. This Guy invented Duel Monsters, but they used REAL monsters. The game was so dangerous it was locked away for a Millenia. Now the magic has been released into Yugi. To transform, Yugi says "YU-GI-OH!!!" And becomes cooler, taller, and gets a deeper voice!