This is shown especially when he is around the player. He is aggressive with others and quite hateful when seen in Snowdin.
His body structure appears the same as his counterpart (besides his teeth). His shorts have a yellow strip instead of a white one and he wears black/red/white shoes or red slippers, as opposed to his counterpart's light blue slippers. His shirt is a red color in contrast to the normal white shirt. Sans wears a black jacket, with the "fluff" at the collar being notably more ruffled than the original Sans'. He wears black shoes with red socks or even red slippers. He wears a black winter jacket with a fur hood and a gray cotton hoodie, and a red sweater with black pants that have yellow stripes going down. His left eye is seen to be glowing red almost constantly, and he has bags under his eye sockets. He is a skeleton with sharp teeth, with one being gold. Underfell Sans looks very similar to his Undertale counterpart. While he respects his brother as his master, he secretly wishes they could at least be more brotherly. Sans hates humanity because they sent the monsters to the Underground, and because of this, he just wants the whole of humanity to perish. He was not previously as mean-spirited as he is now, but since Papyrus became a member of the Royal Guard, Papyrus has been abusing Sans's power and using it mostly for his own cruel, selfish desires. At first, he hated the attitude everyone had towards him, but after a while, he stopped caring. He has had to watch Papyrus kill innocent monsters nearly every day. He is hated by everyone in Snowdin because he is the brother of Papyrus, though he tends to spend his time there anyway, at Grillby's. Some differences about Underfell Sans is that his eye is illuminated in red, which is evil, and that he is destructive. Sans is a skeleton, probably born in Snowdin, who is pushed around by his younger (though significantly larger and more durable) brother Papyrus, to the point of being forced to call him "boss." He is verbally and sometimes physically abused by Papyrus, and is forced to obey his brother's orders. He is the brother of Papyrus and takes on an appearance similar to the Sans of Undertale. Sans is one of the major antagonists of Underfell AU.