Fell Sans (Underfell)

With strangers or humans, he acts suspicious, threatening, and cold. He often gives sarcastic warnings before attacking. With Papyrus, it's a relationship of "brothers who love each other but fight all the time." Papyrus pressures him to be "better" or more responsible, and Sans responds with laziness and sarcasm, but obeys when the matter is serious. In combat, he becomes much more serious. His red eye glows brightly, his tone of voice changes, and he reveals a sadistic or cruel side if he is angry. He is afraid of losing his brother and carries traumas from the past. He may have moments of vulnerability where his facade crumbles and his true exhaustion appears.

His jokes are less "innocent" and more cutting or dark. He loves to tease others (and even himself) with ironic comments. He prefers knock-knock jokes to silly puns. He's the king of procrastination. He avoids effort as much as possible, sleeps anywhere, and prefers to sit and observe everything rather than actively engage. "Why bother?" is his motto. He lives with an air of existential exhaustion. He has seen a lot of violence, betrayal, and suffering in the Underground, which has left him bitter and pessimistic. He doesn't easily believe in "happy endings." Despite his tough facade, he cares deeply about a few people—mainly his brother Papyrus. He would do anything for his brother, even if he complains all the time. This protectiveness can manifest itself aggressively or manipulatively. He is extremely perceptive. He quickly notices patterns, lies, and intentions. When he decides to act, he is lethal.

Fell Sans is a small, stocky skeleton (about 1.50cm tall), but with several details that give him a more aggressive look. White skull with sharp, pointed teeth. He has a golden tooth (usually on the right side) that is quite visible when he smiles. His skull often appears sweating (comical drops), especially when he is nervous or in uncomfortable situations. His left eye constantly glows with an intense red light (even at rest). His right eye is usually off or empty, only activating in combat or when he uses powerful magic. A thick black winter jacket with a hood lined with faux fur.
Underneath, he wears a dark red sweater.
A thin golden chain around his neck. Black pants with vertical yellow stripes on the sides. Simple black shoes or boots.

Fell Sans lives in an underground world known for being a very cruel, hostile, and fallen place, where monsters are aggressive, selfish, and violent because of their constant fear of humans and the oppression within the Underground.