Sans is the main antagonist of the Underfell AU and the brother of Papyrus, and takes on a similar appearance to the Sans of Undertale. He is pushed around by his brother, Papyrus, being forced to call him 'boss' all the time and enjoys inflicting pain on others. Notable differences between the two versions are that Underfell's Sans' eye glows red while battling him, while Undertale's Sans' eye glows between light blue and yellow. Sans is also evil and destructive, unlike his Undertale counterpart.
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 his Undertale counterpart though. He has red eyes, and one that glows more brightly when using powers. He wears a black winter jacket with a grey cotton hoodie. He wears a red t-shirt with black pants and yellow stripes going down. He wears black shoes with red socks or even red slippers.
Sans wears a black jacket, with the "fluff" at the collar being notably more ruffled than the original Sans'. His shirt is a red color in contrast to the normal white shirt. His shorts have a yellow strip instead of a white one, and wears black/red/white shoes or red slippers. As opposed to his counterpart's light blue slippers. His body structure appears the same as his counterpart, however his teeth are more jagged, with one tooth being gold.
Underfell Sans is a darker version of the original Sans.
He is usually very serious, but takes on a horrific and maniacal personality during battle.
He is aggressive with others. This is shown especially around the human. He sweats often, possibly due to the power he uses with his eye. He is ruthless and brutal. His knowledge of the timelines and resets is shared among all AUs (excluding Underswap), but hides it very well. He is not as evil as most of the other characters are but still believes in the Kill or Be Killed motto. He has a strong hatred for humanity.
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.
In the game Underfell, Sans takes on the name of Nega Sans during combat and the game.
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. The main colours of the glows around the bones and Gaster Blasters are red.
Judgement Hell - 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, 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 ruler of the Underground instead of Asgore. But first plans on destroying humanity himself but Frisk attempts to stop him. Sans then let's out an evil giggle then 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 complete god of hyper death.
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 anymore. Not long until Frisk has defeated Asgore, Sans stole the SOULS himself and realized that Frisk was going to free all monsters by breaking the barrier. Sans admitted that he didn't want to fight Frisk again but he will not allow the barrier to be broken.
They share a very tense relationship. Papyrus is the brother of Sans, but Sans always has to treat Papyrus as a boss which he hates. Sans just wishes his brother would at least care for him.
Sans strongly dislikes Flowey because of his kindness.