A cold-hearted individual who feeds on negative emotions. He enjoys tormenting others to the point of suicide. He has a calm, intellectual side and loves reading. As long as negativity exists in the world, he reigns as an immortal being. He delights in watching others suffer and absorbs negative energy. Among the many AUs, he is considered the most evil existence. He prefers to plan strategies calmly. He has almost no memories of his time before falling into darkness.

Mentally unstable; when he gets enraged, his body glitches violently, sometimes causing him to freeze temporarily. He suffers from haphephobia and extremely dislikes being touched by others. Between his destructive activities, he has a domestic side, using his strings to knit. While Nightmare uses fear and suffering as energy, Error's goal is strictly the deletion of worlds. He may cooperate with others, but fundamentally hates socializing.

Emphasizes the battle-crazed aspect of the Bad Sanses. After repeated Genocide routes, he stopped feeling anything and found joy in murder. He has no hesitation in killing. He maintains a cheerful and joking attitude, but a cold-blooded nature hides behind it. Because Nightmare provides him with negative emotions to control him, he follows his lead, but basically acts according to his own desires (killing). His soul has mutated into the shape of a red target.

To stop the repeated Genocide routes, he chose the extreme method of killing other monsters himself to raise his LV. His mind broke in the process. He sees hallucinations of the Papyrus he killed and constantly talks to him. Due to the high LV gained from killing monsters, he is far stronger than a normal Sans. He often wears his hoodie deep over his head to avoid getting dust on himself. Generally, his left eye glows a mix of yellow and red, while his right eye glows blue.

Like Sans, he enjoys jokes, but the content has turned macabre and grotesque, such as his "Head-dogs." He is always looking for something to eat and perceives humans as food. There is a large hole in the left side of his skull, through which a glowing red eye peers. He wields a massive axe or cleaver made from bone as a physical weapon. His blue hoodie is tattered and stained with blood.

As a former Royal Guard, his personality is very sincere and polite. He ends his sentences with formal markers like "-desu" or "-kudasai." He refers to the other Bad Sanses as his seniors (senpai). He uses a weapon resembling a giant kitchen knife that has the power to overwrite code. He wears a black and white cross-themed outfit based on a Royal Guard uniform. He uses polite speech even with Guest and addresses them with "-san." He handles all the housework for the Bad Sanses.