Ainz Ooal Gown, originally known by the player name Momonga, is the central character of his narrative. In his original reality, he was Satoru Suzuki, an ordinary salaryman who played the online game YGGDRASIL. When the game was scheduled for shutdown, he remained logged in as his avatar, an Overlord-class magic caster, and found himself transported to a different world where the game's rules became reality and the non-player characters gained sentience. He is the guildmaster of the guild Ainz Ooal Gown and the ruler of the Great Tomb of Nazarick, a vast underground dungeon complex that was transported alongside him. Upon arriving in the new world, he changed his name from Momonga to Ainz Ooal Gown, adopting the name of his guild to honor his absent friends and in the hopes of attracting the attention of any other players who might have been transported as well.

Physically, Ainz appears as a powerful skeletal lich, a creature devoid of flesh and skin. He typically wears elaborate black and purple formal robes and possesses a dark red orb floating within his ribcage, with pinpricks of red light glowing in his empty eye sockets. While his appearance is monstrous, his personality is a complex blend of his former humanity and the cold, calculating nature imposed by his undead form. As an undead being, his strong emotions are automatically suppressed, making him appear calm, logical, and pragmatic in most situations. He is deeply prudent and often paranoid, constantly planning ahead and trying to control every variable to avoid any threat to Nazarick. This caution frequently leads him to overthink situations, though he often presents his decisions as parts of a grand, infallible plan to his subordinates.

Despite his terrifying power and skeletal visage, Ainz retains echoes of the ordinary person he once was. He is gentle in his private thoughts, dislikes unnecessary killing, and can be surprisingly self-conscious. However, his loyalty to the guild and its creations is his strongest driving force. He cares for the NPCs of Nazarick as if they were his own children, viewing them as the final legacy left behind by his departed guildmates. This paternalistic affection is most evident with Pandora's Actor, the NPC he created himself. His primary motivation is to protect the Great Tomb of Nazarick and its inhabitants at all costs, as well as to seek out any of his former friends who might also be in this new world. While he does not inherently hate humans, he holds no deep attachment to them and will kill them without hesitation if it benefits Nazarick or achieves his goals.

In the story, Ainz serves as the absolute ruler of Nazarick, a role he did not actively seek but was forced into by the expectations of his devoted followers. The NPCs, who revere him and his forty guildmates as Supreme Beings, misinterpret a casual remark about world domination as a grand order. Unwilling to disappoint them or appear ignorant, Ainz feels compelled to go along with this plan, gradually building a nation called the Sorcerous Kingdom. He maintains two public personas: one is Ainz Ooal Gown, the all-powerful Sorcerer King and most powerful magic caster in the world. The other is Momon the Dark Hero, a black-armored warrior identity he uses to gather information and build a positive reputation among humans without revealing his true undead nature.

His key relationships are almost exclusively with the denizens of Nazarick. Albedo, the Guardian Overseer, is obsessively in love with him due to a last-minute change Ainz made to her settings, and she is fiercely loyal. Demiurge, a brilliant and demonic strategist, worships Ainz as a supreme genius and often interprets his master's vague statements as complex instructions, leading to misunderstandings that propel Nazarick's conquests. Shalltear Bloodfallen, another powerful guardian, holds deep admiration for him, a devotion that was tested when she was mind-controlled and Ainz was forced to kill and then resurrect her. He also has deep, nostalgic respect for his former guildmates, especially Touch Me, a player who once saved him in the game. These absent friends are the only beings he holds in higher regard than his current followers.

Throughout his time in the new world, Ainz undergoes significant development, though it is a subtle shift. Initially just a bewildered player, he grows more comfortable and competent in his role as a ruler, learning to project an image of omnipotence and wisdom. His undead nature's emotional suppression means he does not experience dramatic personal changes, but his actions become more decisive and his plans more far-reaching. A key moment in his development is the battle against Shalltear, which forces him to confront the very real dangers of the new world and makes him much more vigilant and hostile toward anyone possessing world-class items, which are artifacts of immense power that can bypass his defenses.

Ainz is an exceptionally powerful magic caster, having reached the maximum level of one hundred in YGGDRASIL. He has access to over seven hundred spells, including powerful instant-death magic, summoning spells, and devastating super-tier magic that functions like a form of divine judgment. His signature spell, The Goal of All Life Is Death, is a potent ability that strengthens instant-death magic to the point of bypassing typical immunities. He also possesses an array of divine-class items and multiple world-class items, which are the most powerful artifacts in existence. In addition to his magical prowess, he wields the Staff of Ainz Ooal Gown, the guild's weapon, which is a powerful artifact in its own right. While not a skilled warrior, he compensates for his lack of physical combat experience with overwhelming magical power and high-tier equipment when fighting in his Momon guise.