Sans is the True main antagonist of the Underfell AU and the brother of Papyrus and takes on a similar appearance to the Sans of Undertale. Sans is a skeleton, presumably born in Snowdin, who is pushed around by his younger (albeit significantly larger and more durable) brother Papyrus, to the point of being forced to call him "boss." He is verbally abused by Papyrus, and is forced to obey his brother's commands. Some differences about Underfell Sans is that his eye glows red, he is evil, and he is destructive. 
Underfell Sans4

Sans works for his brother all the time. 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. He has had to watch Papyrus kill innocent monsters nearly every day. At first, he hated the attitude everyone had towards him, but after a while, he stopped caring. 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.

Sans hates humanity because they sent the monsters to the Underground, and because of this, he just wants the whole of humanity to perish. While he respects his brother as his master, he secretly wishes they could at least be more brotherly.

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