The life of Yugiohtep I, or Yugi Motou as he is now known, is the opposite of his counterparts in other uiniverses. Whereas in most universe he is the boy of the present, the "aibou" to Atem's "mou hitori no boku" from the past, in his reality, their roles are switched, and it is Yugi who is "mou hitori no boku", or as some others have called him, "the nameless pharoah", "the other Yami", or (humorously) "Yami no Yami".
His full title being Kanakht Khaemwaset Sekhempahtydsejerkhaw Nebtawykhamerkhe za Yugiohtep Neferkhe, he was born to Akhinaden, the pharoah of Egypt, into a life of privilege and power. As the first born, as well as the son of the great royal wife, he was destined to suceed his father. He did so when he was 13, when his father died under mysterious circumstances tied to the power of the Millennium Items. Though he feared the day he was crowned, in the end he was crowned and on the same day he married and bedded his first wife, Mananebtytari, known affectionately by the boy as "Mana".
The Millennium Items were forged by the founder of their dynasty, the Unnamed Pharoah, who sacrificed 1,000 men to the god of darkness, Zorc Necrophades, to gain more power for himself and his priests. However, though the items gave their users enhanced powers and stronger spirit personifications - known as ka - the items were cursed. The users of the items were all destined to die when their souls were taken to Zorc by his servants, which added to the demon's power. Likewise, those who sold their soul to the service of Zorc ehnanced their ka with darkness, but lost half of their ba - the spiritual longevity of their lives. In the end, all fell into darkness.
To this end, Yugi was given his father's item, the Millenium Pyramid, a trinket which, along with the basic powers each Item gave its user, gave him the ability to see and seal memories. He always knew that he would one day have to face a servant of Zorc's, as they always attempted to challenge the item bearers to ka duels. But he didn't know that Zorc, strengthened by years of soul-gathering, had decided it was time for him to fully awaken, and to do so he would need all seven items returned to their birthplace - the tomb of Kul Elna where the Nameless Pharoah had forged the pact with blood. To this end he appointed a servant that would be well-suited for such a task - a descendant of the sole survivor of Kul Elna, the Thief King Bakura.
One by one the Items were taken from their owners, who were all nobles and advisors to the pharoah - Akunadon (Yugi's uncle, who, it turned out, was being controlled by Zorc and was killed to save him), Shada, Isis, Set, Kalim, Mahad (who fused himself with his ka to become the Black Magician, thereby sacrificing his life to save Yugi), and even Yugi himself - and the demon was resurrected and Bakura killed. It ravaged the countryside, killing all in its way. One by one the advisors were killed, their powerful ka swept aside mercilessly until only Set and Yugi were left.
At this point Yugi remembered that his item had the ability to control memories and as such was extremely powers. Perhaps it was possible that - in exchange for his memories - he could seal the creature away within the Pyramid, using his own memories to ensure it wouldn't be able to escape. And so, using his name as the key, Yugi managed to free most of the spirits trapped within the monster, and he sealed himself and the spirit of the monster inside the Pyramid, instructing Set to shatter it to prevent it from being unleashed. Set did that and more, effacing Yugi's name from every monument in case someone managed to reassemble the Pyramid.
Though the box holding the Pyramid's remains was passed down for several generations, it, along with all of the other items, subsequently disappeared through neglect, invasion and by the ravages of time. The Pyramid ended up in a mass peasant tomb some ways away from where Yugi's actual funerary chamber was - though this was also lost to time. Soon Yugi became another Nemless Pharoah, just as the creator of the Items was, and to future generations it was as if he had never existed.
Meanwhile, millennia passed, and gradually Yugi forgot everything about who he was, where he came from, and even his own name and the meaning of his own existance. Though naturally optimistic, even he eventually despaired that anyone would ever be able to free him from his prison, which was filled to the brim with darkness and hatred. He passed the time creating games, in particular the Shadow Game, which was much like the duels between the agents of darkness and the priests, save that there were penalties for cheating. He played with the shadows until finally, a portal opened and sucked Yugi in....straight into the mind of Yami Motou.
Yami Motou was a Domino City loner, a bit anti-social and dark - but he loved games ever since he was a child. Despite the front he put up as a child who loved black eyeliner and hated authority, the truth of the matter was that Yami was painfully shy, only able to express himself while playing games - and due to a birth defect his eyelashes were a mutant white. He found the Millennium Puzzle, as it came to be called, in the back of the family game shop, and heard the legend from his grandfather that whoever solved it would have their greatest wish come true. Yami's wish was that he could have friends, and for eight years he labored on the Puzzle - until finally, after being attacked by a bully for daring to stand up in defense of two gangbangers, Katsuya Jounouchi and Hiroto Honda, he finished it. The Shadow Games were unleashed once more onto the world.
At first, Yami was unaware that Yugi on occassion took control and imposed penalties on people who threatened him and his friends - and Yugi was at first unaware he was anything more than a part of Yami's mind. Gradually, however, the two boys began to realize each other's nature, and to that end Yami, happy that he had finally found a friend, agreed to help him find his memories, calling the phariah his other self - "mou hitori no boku". In return, Yugi gave Yami, whom he called "aibou", considerable knowledge of the Shadow Games once played in Egypt, which were similar to the up-and-coming game known as Magic and Wizards.
It was only natural that both Yami and Yugi came to master it, and with each passing duel Yugi began to regain a small portion of his memories. The events of Duelist Kingdom and then Battle City revealed to Yugi and Yami the seven Items, the three God cards which in fact represented Yugi's unique ka as a pharoah of Egypt, and the stone frieze which allowed entrance to Yugi's true memories in the place known as Memory World.
In Memory World, Yugi finally regained his memories, but it was only with the help of Yami, Jounouchi, Honda and Anzu Mazaki (a popular girl at school who, incidentally, Yami had a crush on) that he regained his name. With it he managed to unlock the ultimate ability of his ka, the merging of the three Gods to create a being known as the Creator of Light. With it he destroyed Zorc, who needed to kill Yugi in order to free himself from the Puzzle and destroy the world, and, to everyone's surprise, restored his body completely, as well as the spirits and memories of those who had been sacrificed to Zorc.
It turned out that Yugi's selfless act of sacrifice of sealing himself into the Pyramid had pleased the gods, the Creator of Light revealed, and so they preserved his body so he might once more walk among the living to live out the rest of his natural life. Mahaado's spirit was also restored, though because he had combined his ka and ba he was doomed to remain as a duel monster; he chose to stay by his king's side and thus live with him and Yami. After this was revealed to him, the Creator of Light transformed into a duel card, which went into Yugi's deck. In that way the power of the pharoahs, both before and after him, could be utilized in duel.
Yugi was glad that he finally got his name and life back, but more importantly he could stay with Yami and his friends. He could finally see what life in Yami's world outside of dueling was and have fun like a normal kid could. Just as Yami had sat him down to start explaining the world of this millennium, however, Yugi suddenly blacked out....
...and woke up in Econtra.