Yami Yuugi is the spirit of the mighty Pharaoh who once ruled Egypt 3,000 years ago. To save the world, he sealed evil within the Millennium Puzzle, but he had to seal his own soul too. The puzzle was broken apart and remained unsolved for thousands of years, until destiny brought the puzzle to Mutou Yugi. He was able to piece together the complicated puzzle and awaken the spirit of the Pharaoh that resided within.
Of course, he has no recollection of his days as the Pharaoh, and his past was one of the biggest mysteries of the series. At the beginning of the Battle City arc, he visits the Domino Museum and learns from Ishizu about his past as the Pharaoh, and that the only way to unlock his memories is to collect the keys to his memories - the three god cards and the seven Millenium items - in Kaiba's upcoming Battle City Tournament.
Past & Present
Eventually the series turns its focus on the Duel Monsters card game, and Yami Yuugi's first major opponent in the Duelist Kingdom arc is Pegasus J. Crawford, the multi-millionaire who created the modern Duel Monsters card game. Pegasus had the Millennium Eye and wanted Yugi's Millennium Puzzle for himself, so he gave Yugi no choice but to enter his tournament with his grandpa's soul at stake. Yami Yuugi is able to face off against Pegasus and win.
In the final arc of the series, Pharaoh's Memory World, Yami Yuugi heads to the ancient tablet with all the god cards and Millennium items and is thrown into the Memory World. He relives his life as the Pharaoh and meets the strong and faithful Priests and Priestess that made up his court, learns how the Millennium Items were forged, and discovers his true name.
Yami Yuugi is confident, brave and honorable, but can also be mysterious and distant. Shadi once entered Yugi's mind after Duelist Kingdom and got lost in the confusing maze of illusions, trying to find the room to Yami Yuugi's closely guarded soul.
He is very protective of little Yugi, whom he calls aibou (partner). When he senses danger, he appears and takes over Yugi's body - especially during duels - and takes care of the situation. The poor guy is also constantly targeted by villains - Pegasus from Duelist Kingdom, Marik from Battle City, Dartz from Doom, Thief King Bakura, and Kaiba throughout the series. It's not easy being a Pharaoh and the King of Games.
Since his true name was not known until the end of the series, he went by quite a lot of names. Below are the most common ones:
» Yami no Yuugi (Dark Yuugi) - Used during the first series
» Mou hitori no boku (the other me) - Only used by Yuugi
» Mou hitori no Yuugi (the other Yuugi) - Used by his close friends Jounouchi, Anzu, and Honda, who realized there were "two" of them
» Namonaki Pharaoh (the nameless Pharaoh) - Used by those who knew about his true identity, later in the series
» Yuugi - Used by everyone else
Atemu vs Atem? There is some dispute over the spelling of his name. Officially, his name is spelt アテム (Atemu). However, because of the way romanization works in the Japanese writing system, the last "u" may or may not be intentional. Basically, there is no way to spell Atem in Japanese, so the closest thing is Atemu. Therefore, both spellings are technically correct. I prefer to stick to the Japanese pronunciation, so I spell it Atemu on this site.
English dub: He is simply called Yami Yugi, or Yami for short, in the dub versions. Later in the series, he is called Pharaoh Yami, Nameless Pharaoh, or just Pharaoh.