His blue jacket has taken on a slightly greenish tint, and he has bright red pupils. A Sans mutated as a result of the player scrambling his data. He can do things that should be impossible, such as grabbing the player's slash effects with his bare hands or nullifying attacks.

The area around his eyes always bears a wide, eerie black grin. He harbors two personalities, "Sans" and "Chara," which swap randomly. When in Sans' personality, he is lazy and makes self-deprecating dark jokes about his miserable state. When in Chara's personality, he becomes extremely cold, intelligent, and sociopathic in his actions.

He has made a pact with a demon, and half of Sans's head (face) is violently shattered, with a mass of unsettling yellow and light blue eyeballs exposed from within, giving him a grotesque appearance.

Instead of the usual blue hoodie, he wears a raincoat to shield himself from the rain. The color scheme is overall dark, with an ominous palette of black, dark gray, or stained blood red. When his true nature is revealed, he has a mouth split wide open and a crazed expression. His eyes glow an eerie red, and blood or black liquid appears to stream from them like tears.

He is far taller and more powerfully built than the original Sans, with a robust, sturdy physique. He also tends to be somewhat hunched over. He has an enormous jaw and huge, sharp teeth, with his lower jaw split clean in two. There are large cracks in his head and skull, from which orange and blue energy constantly flows. When enraged, steam bursts from these cracks with a hissing sound. His pupils glow ominously in two colors, orange and blue. His limbs are extremely large, resembling massive, sharp claws. His bones have a moldy, greenish tint. He wears a blue hoodie similar to the original, but he does not wear slippers or any shoes, remaining barefoot. He is driven by an instinct to mercilessly kill the player, but he is still capable of conversation.

His pupils are tiny like pinpricks, and his mouth has an unnaturally raw shape with exposed gums. He is far taller than the original Sans and wears a familiar hoodie, but it is sleeveless, and he does not wear his trademark pink slippers, remaining barefoot. Inside his chest are packed the agonized, twisted souls of countless victims. He was originally not a monster, but the end result of a man named Samuel, who committed heinous crimes repeatedly in the real world and was feared as the "Worst Human" and a "Monster." He is cold and sadistic, enjoying inflicting mental and physical torture on humans to make them suffer. Furthermore, he is an astral body, making him immune to physical and magical attacks.

Normally, he looks no different from the original Sans, but when his true nature is revealed, his legs stretch out and his arms extend until they touch the floor, his face turns black except for his eyes, and occasionally an eerie mouth emerges.

His base design is a combination of a skeleton like Sans and a Siren Head, with a body covered in withered tree branches and roots entwined around it. Two sirens shaped like Gaster Blasters protrude from his head. Normally, he would be about 21 meters tall, but this individual is 41 meters.