Fell Sans

Sans is the main antagonist of the Underfell AU and the brother of Papyrus and takes on a similar appearance to the UnderTale Sans. 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 destructive, unlike his UnderTale counterpart.

Appearance
Underfell Sans looks very similar to his UnderTale counterpart. He is a big-boned skeleton with sharp teeth, with one being gold. He is slightly taller than UnderTale Sans, though. He has red eyes, and one glows more brightly when using his powers. he wears black sweatpants that has a vertical yellow stripe on each leg. He wears black/red/white shoes with yellow or red socks, and possibly red slippers. He has bags under his eyes.

His shirt is red in contrast to UnderTale Sans' white t-shirt. His shorts have a yellow stripe on each leg opposed to UnderTale Sans' Pants that has a white stripe. he wears red/white shoes, as opposed to UnderTale Sans' pink slippers. He wears a black coat that has a golden zipper and Tan fluff on the hood. His body structure appears ALMOST the same as the original Sans. He is sometimes seen as sweating possibly from overusing his powers.

He is sometimes portrayed wearing a red collar and having a crack in his skull (and sometimes with fluff on the sleeves, which is not really Canon.

Personality
Underfell Sans is a darker and edgier version of the original Sans.

He is aggressive with others. This is shown especially around the human. He sweats often, possibly due to his overuse of his powers. He is ruthless and brutal. His knowledge of the timelines and resets is shared among all AUs (excluding some interpretations of UnderSwap), but hides it very well. He is not as evil as most of the other characters are, but still believes the "Kill or Be Killed" motto. (In comics and in Fanon, He does say curse words most of the time.)

This version of Sans makes appearances in both the Undertale Comics, where Sans appears as one of the three bosses along with his brother Papyrus and Asgore.

Powers and Abilities
Sans is a little more powerful than his UnderTale counterpart. He can manipulate bones and telekinetically move a living soul at ease. He can also manipulate Gaster Blasters. He can Teleport like Undertale sans. The bones and his Gaster Blasters he summons are red.

In Battle
Judgement Hall - In the True Pacifist timeline, Sans keeps an eye on Frisk. He hates how Frisk keeps being nice and waits at the Judgement Hall. This fight would be similar to the boss fight in UnderTale, only more brutal, harder and deadly. Once Frisk managed to 'reform' Sans, he refuses to fight anymore.

Omega Sans - After their encounter in the Judgement Hall in the genocide timeline, Frisk will still have to encounter Asgore. As Frisk and Asgore are distracted, Sans sneaks in and steals the human souls. After Frisk spares Asgore, Sans killed Asgore with his Omega Gaster Blaster (or maybe just confront them). He is now a God-like monster, containing the six human souls. He admits that he doesn't want to fight Frisk again, but refuses to let monsters go free. Now that he is a god, he will now be the ruler of the Underground instead of Asgore. He first plans on destroying humanity himself, but Frisk attempts to stop him. Sans then let out an evil giggle and calls Frisk an idiot and the fight begins. This fight would be similar to the fight in UnderTale with Omega Flowey and Asriel Dreemurr, the God of Hyperdeath.

(unsure if that is Canon) ^

Frisk:
Pacifist route:

Sans has a personal hatred for Frisk and wants nothing more than to kill Frisk themself. After their encounter in Judgement Hall, Frisk managed to 'reform' Sans. Sans then said he wouldn't harm Frisk any longer.

Genocide route:

Sans first assumes that Frisk is weak and will eventually be killed. After losing his brother, Sans becomes broken. He doesn't feel hatred since, in this world, monsters follow the motto "kill or be killed." Sans then goes to the Judgement Hall and waits for Frisk. Instead of fighting or threatening Frisk, he instead thanks Frisk for believing in the motto. Sans is reluctant to fight Frisk because of how powerful Frisk became. Sans tries to calm Frisk down, but to no avail. When it is clear that Frisk will not back down, Sans begins to fight with his full strength, but in the end, Sans dies after 24 rounds. During the fight, Sans loses his fear of Frisk and comes to terms with his imminent death.

Papyrus:
They share a very tense relationship. Papyrus is the brother of Sans. Their relationship is based almost solely on conflict and pain, such to the point that Papyrus forces Sans to call him 'boss' rather than 'bro', which he hates. They rarely do speak to each other as equals and Papyrus always speaks before him and is attributed the merit sans what has dong among them of the puzzles that are seen in Snowdin. It is said that Papyrus is only so hard on him to keep them both safe. Papyrus likes to bully and even abuse sans. Sans gets anxious near Papyrus. Sans wants his brother to at least take care of him, even though some speculate that Sans battered Papyrus as a child.

Flowey:

Sans Hates Flowey because of his Frailty.

Fun facts:
 * In the game Underfell, Sans takes on the name of Nega sans during combat and the game.
 * He is allowed to say curse words.