Pharaoh Atem is the ancient Egyptian ruler whose spirit was sealed within the Millennium Puzzle, an artifact that would later be solved by the teenager Yugi Muto. Once the puzzle was completed, Atem's spirit awakened and took on the identity of Yami Yugi, sharing Yugi's body as a separate consciousness. In life, Atem was the son of Pharaoh Aknamkanon and was born during the Eighteenth Dynasty of Egypt, shortly before the creation of the seven Millennium Items. Those items were forged through the sacrifice of the people of the village of Kul Elna, a dark legacy that shaped the history of his kingdom. Atem ascended to the throne at a young age and became known as a kind and courageous ruler, but his reign was threatened by the great evil Zorc Necrophades. In the final battle, Atem used his own name as the key to a powerful spell, sealing both Zorc and his own soul within the Millennium Puzzle and erasing his memories so the seal could never be undone. His priests then removed all records of his name, leaving history to remember him only as the Nameless Pharaoh.

In the modern era, Atem's personality stands in clear contrast to Yugi's. Where Yugi is gentle, timid, and hesitant, Atem is confident, assertive, and commanding. He speaks with authority, carries himself with regal poise, and displays sharp strategic thinking. He is fiercely protective of Yugi and his friends and is willing to place himself in danger to keep them safe. Atem values honor, fairness, and the bonds between people, and he treats dueling as a test of character as much as skill. Early in his existence, he was far harsher, acting as a vigilante who punished wrongdoers through Shadow Games and Penalty Games that could inflict psychological or spiritual harm. Over time, he became less severe and more merciful, particularly after Yugi challenged his willingness to destroy an opponent, and he developed a deep sense of empathy and grief as he confronted the fragments of his forgotten past.

Atem's central motivation is the recovery of his lost memories and his true name. This search drives him through the major tournaments and conflicts of the story, as he learns that the Egyptian God Cards and ancient tablets hold the key to his identity. Alongside this personal quest, he is motivated by loyalty to his friends and a sense of duty to protect them from the dark forces drawn to the Millennium Items. His role in the story is that of a guardian and champion: he emerges to face dangerous opponents, solves deadly games, and ultimately confronts the ancient evil that once destroyed his kingdom. He is also the driving force behind the resurgence of Duel Monsters as a world-shaping game, using his mastery to defeat rivals and uncover the truth of his past.

Atem's most important relationship is with Yugi Muto, who calls him the other me. The two share a body and a deep bond, with Atem acting as protector and mentor while Yugi grows into a confident duelist in his own right. Their friendship is tested throughout the story and culminates in the Ceremonial Battle, a final duel in which Yugi proves himself worthy of the title King of Games and Atem accepts his rest. Atem also shares strong connections with the other members of Yugi's circle, including Joey Wheeler, whose courage and growth Atem respects, and Tea Gardner, whose kindness and strength he values. His relationship with Seto Kaiba is that of a fierce rivalry, built on mutual respect and a desire to prove superiority; Kaiba refuses to accept Atem's eventual departure. In the ancient past, Atem was served by Mahad, his loyal magician whose soul became the Dark Magician, and he worked alongside Priest Seto, the ancient counterpart to Kaiba, as well as other priests who wielded the Millennium Items. His chief antagonist in the ancient world was Thief Bakura, the survivor of Kul Elna who sought revenge and the revival of Zorc.

Atem's development is one of the most significant arcs in the story. He begins as a mysterious and nearly invincible figure who dispenses justice without hesitation, but he gradually becomes more human. He struggles with fragmented memories of a life he cannot recall, experiences doubt and guilt, and at times acts arrogantly, as seen when his pride leads him to lose a duel he should have won and cost his friend dearly. These failures teach him humility and reinforce his reliance on the people who support him. When he finally travels into the Memory World and relives his final days in Egypt, he recovers his name and learns the full truth of his sacrifice. Having accepted his past, he chooses to pass into the afterlife rather than remain bound to the mortal world, entrusting his legacy and his friends to Yugi.

Atem possesses a wide range of notable abilities. Through the Millennium Puzzle, he can initiate Shadow Games, magical contests whose rules bind all participants, and impose Penalty Games on those who lose or break the rules. The puzzle also grants him the power to sense thoughts, emotions, and the presence of other Millennium Items, to communicate with Duel Monster spirits, to cast illusions, and to protect himself and others from magical harm. He is able to switch consciousnesses with Yugi, taking control of their shared body when needed. As a duelist, he is a brilliant strategist capable of reading opponents and turning the flow of a game, and he is closely associated with the Dark Magician and the other monsters of his deck. He is one of the few characters able to control all three Egyptian God Cards: Slifer the Sky Dragon, Obelisk the Tormentor, and the Winged Dragon of Ra. In ancient Egypt, he used a sacred duel disk called the DiaDhank to summon spirit monsters through his ka, and at the climax of the ancient conflict he fused the three Gods using his true name as the key to create the Creator of Light, Horakhty, and destroy Zorc. His signature approach, often described as trusting the heart of the cards, reflects both his supernatural connection to the game and his belief in the strength of his bonds with others.