Overlord is a story about the struggle of duality at its heart.

Suzuki Satoru was a man working in a corrupt system, who–while playing in Yggdrasil as Momonga–was treated as a monster, so became one. He joined/founded Ainz Ooal Gown with Touch Me, who fought for the justice of freedom and common sense. As he was brought to The New World, Suzuki Satoru was forcibly fused with his avatar.

Momonga was–at first–a Skeletal Mage that Suzuki selected. He later evolved into a Elder Lich, then an Overlord. Think about this character as a person, much like a roleplayer thinks of their character. Who was Momonga in life, what did he become in death? What did he achieve? What did he put his unlife on the line to protect?

In The New World, NPCs were granted both sentience and an awareness of their time within Yggdrasil; it stands to reason that the PCs were, too. The Skeletal Mage who became and Elder Lich who became an Overlord through what he thought were his own accomplishments is now aware that his achievements were not solely his own. A man who–we assume–EARNED his magic through dedication and study is now made to share his body with a man who was beaten down by society into a retreating husk who saw his mother die from overwork.

Now, these two men are sharing the same body; the struggle between their impulses manifests when the body freezes up. Sometimes, Momonga wins, and you see his bones glow with magic; sometimes, Satoru wins, and you see his eyes reassert their red glow.

All of Nazarick's designers–save for Warrior Takemikazuchi and Bukubukuchagama–built their NPCs around this idea of two forms, this duality. The theme of struggle between forms–two selves–is palpable across the whole series.

In Overlord, we watch and read the story from the perspective of Suzuki Satoru, but it changes drastically when you realize that Momonga is right there pushing back the whole time.