The Pharaoh formerly known as Nameless
The idea of Dark Yuugi being the spirit of an unknown Pharaoh was introduced early in the Battle City arcs of both the anime and the manga. Both Dark Yuugi and Yuugi Mutou, his host, readily accepted the idea once they saw a series of tablets at the Domino Museum, as shown to them by Isis Ishtar. In order to find out more about the past, Isis told Yuugi that he would need the God Cards, and he would have to defeat her brother Malik Ishtar, to gain possession of them all. She did not tell Yuugi that she herself had given one of the God Cards to Yuugi's archrival, Seto Kaiba!
At the end of the animated Battle City arc, Yuugi did emerge triumphant, but when he took the cards to the museum, Dartz took control of them and stole them away. Two more filler arcs (the Orichalchos Saga and the KC Grand Prix) would occur before the truth about Dark Yuugi's name would be revealed in the fifth animated arc of the Yu-Gi-Oh Duel Monsters series.
The Great Name Debate
Toward the end of the Memory World arc (also called the Ancient Egyptian arc, the Dark RPG, and many other names), Dark Yuugi's name as Pharaoh -a critical piece of information both personally and for the sake of a Dark Duel that Dark Bakura instigated- was revealed.
The hieroglyphs roughly pronounce "Ah-tem-oo" which would translate to "Atemu" in Japanese katakana. However, historically speaking, very few royal names end in vowels such as 'u' (though many of them start with vowels such as 'a', and a few end with 'i'). In actuality, ancient Egyptian hieroglyphs had implied vowels, where very few pictographs actually represented what we know as vowels in the English language.
One could argue that regardless of where series creator Kazuki Takahashi knew anything about ancient Egyptian royal names, he still added the "chick" hieroglyph that indicates an "oo" sound, so the Pharaoh's name must be "Atemu." However, Takahashi knew he was writing for a Japanese-reading audience, and it would be much easier for them to pronounce a name that included many vowels (since the Japanese language is based off syllables, all of which have vowels in them except for one). This is why names such as Isis (the original Egyptian name) get changed into Aishizu (in the Japanese), and the dub reads the romanization of that name incorrectly (producing Ishizu, pronounced Ee-shee-zoo, rather than the Eye-shee-zoo of the Japanese).
Furthermore, both the dub animation and the translated manga (published in English by Viz Media) use the name "Atem" rather than "Atemu."
The name "Atemu" may or may not be related to a real name in ancient Egyptian mythology. Depending on which version of the Yu-Gi-Oh series you are familiar with, the time from which the Pharaoh comes is varied, and anyone interested in ancient Egyptian history could probably tell you that there are elements from several different time periods and dynasties all mish-mashed into the Yu-Gi-Oh series to make it more attractive.
One such example is the name "Aten," a name for the holy Sun Disk that the pharaoh Akhenaten worshipped. He changed the state religion of Egypt from the pantheon of Gods (Ra, Isis, Nephthys, Osiris, Horus) to that of the single god Aten.
The name could also be related to "Atum," considered one of the creator gods in the Egyptian pantheon. Atum also goes by "Atem," and is sometimes considered the father of Osiris, the Lord of the Underworld in Egyptian myth.
There is also mention of an Egyptian warrior by the name of "Atem," although not much is actually known about him.
Finally, the duels between Seto Kaiba and Dark Yuugi may in fact have their own ties to the fights between the King of Earth, Osiris, before he became the Lord of the Underworld. Supposedly, Kaiba's previous incarnation Priest Seto may have had his named inspired by the Egyptian god Set, Osiris' brother. Set sought after Osiris' crown, and used a great deal of trickery and battling to gain the crown. He never did truly gain the crown, but he was revered and worshipped in his own way. He was partially responsible for the death of Osiris, who ruled over the Underworld (displacing Anubis, who later became known as the God of Mummification).
Birthdate: Unknown, however it is known that he was approximately 15 when he first became Pharaoh after his father's death, and that his cousin, Priest Seto, is at least a few years older than him (as Seto was a child when Atem was just a baby).
Height: See Dark Yuugi
Weight: See Dark Yuugi
Hair: Red, yellow (somewhat gold) and black
Eyes: Tends to change from crimson (a blood-like red) to dark violet
Relatives: Akunamkanon (father; deceased), Priest Seto (cousin), Akunadin (uncle though it's never mentioned in either version whether Atem knows of his relation to Akunadin but most likely does as he told Seto Kaiba Priest Seto's reincarnation about Akunamkanon.), mother (deceased)
Ka: Osiris, Obelisk, Ra
Voice Actors: Dan Green (dub), Kazama Shunsuke (Yu-Gi-Oh Duel Monsters).