Fell sans

Mistakes!: Sans is not rude, he does not use swear words or obscene gestures.
The name "Edgie" is never mentioned, and the name "Red" is not officially canon.
Papyrus doesn't treat Sans like a slave or a dog.
For this reason, Sans does not wear a collar. Frisk does not wear a black sweater with red stripes.
Mettaton only has two arms in his normal form, and his body is not square.
Napstablook is not a black figure with red eyes and a mouth.
The paths in Underfell are not the same as in Undertale.
Mettaton is not hated.
Papyrus is not the head of the Royal Guard, that role still falls on Undyne's shoulders.
Therefore, Undyne did not leave the Royal Guard.
Asgore still carries a trident, rather than a cleaver or scythe.
A few things that are more forgivable, but should be kept in mind when trying to follow canon: Sans wears a long-sleeved t-shirt/shirt under his jacket, rather than a sweater.
Sans's fur on his hood isn't yellow.
Sans doesn't have fur on the sleeves of his jacket.
Sans and Papyrus' pupils are not red.
Sans can't turn his eye off. His eye also doesn't shift between yellow and red or purple and red; it shifts between orange and yellow, or remains red.
Sans and Papyrus don't have skull cracks in their current designs.
Sans's boots have slits in them and he doesn't wear slippers.
Sans doesn't always call Papyrus boss.
Alphys's glasses are not yellow and her eyes are not the same as the glasses.

Gaster
Gaster was probably the last hope that the monsters would emerge from underground without resorting to killing each other or humans. But he reached a dead end and eventually disappeared.
Gaster was more of a misunderstood scientist, dismissed as crazy, idiotic, or perhaps evil. He, too, had been affected by the war, but he tried his best to bring it all back, ultimately failing.

Chara
Chara doesn't hate humanity, but he doesn't like it either. He knows what humans have done wrong, but he also understands that monsters have hope for survival on the surface. He's sweet and caring, and puts the monsters' feelings above his own.

Azriel
Azriel is morally confused, so he can be rude at times. He's a bit arrogant, but if you convince him of something, he'll understand.

At first, Asriel was very wary of Chara's intentions—he was distant until he saw Chara helping Toriel. He died at the hands of humans when he came to the surface to take Chara to their favorite flowers.

Alphys
Her personality has also been negatively affected. She wears a faded white coat with a red, black-striped sweater and a black dress underneath, as well as glasses with a strange pattern, a gift from Undyne. Alphys deliberately made Mettaton's EX form the least pleasing to him in order to create the ultimate body, dangerous in both design and power. She lied to him that he would get the body of his dreams if he helped her convince Asgore to let her become the Royal Scientist. She is also insane and experiments on absolutely everyone. She is extremely confident, thinks only once and chooses immediately. Almost as arrogant as Asgore. A fashionista.

Sad Mannequin
The Mad Mannequin is not mad, but sad in Underfell.

Mettaton
Still famous and beloved by all, albeit at the cost of his own sanity. Alphys has forcibly programmed his body to malfunction at any emotional displays of remorse, anxiety, or defiance. Despite this, he has masterfully mastered the art of feigning confidence, though he can't seem to shake the menacing smile from his face. Even more dramatically, he faints and giggles.

Mettaton has an extra pair of arms that Alphys can control if the robot manages to bypass the program. Having been repeatedly lulled into a false sense of friendship, he does the same to Frisk, convincing them they're on his side before betraying them.

He has the highest heels of his boots and may also be the first or second tallest of the main characters,

Papyrus
Unlike the canon version, he doesn't try to join the Royal Guard, as he already is. He wears black armor, black jeans with a red belt and a gold buckle, a red scarf resembling a torn cape, and elbow-length red gloves. Like his brother, his teeth have become sharper and have become part of his skull, while his bones have acquired a gray tint. He is taller than the original Papyrus, and the red heeled boots he wears, a gift from Alphys, make him even taller, making him the third tallest of the main characters.

He's more nervous and angrier than the original Papyrus, though kinder than other monsters. Like other monsters, he's killed, but he deeply regrets it. He hates Asgore more than any monster, but his respect for Undyne mitigates his emotions. Unlike in canon, he seems to care for his brother even more, telling him "not to be ashamed to complain when someone hurts him." He's a fashionista. He loves mean puns and doesn't understand regular ones. Instead of spaghetti, Papyrus makes lasagna, and it's delicious. The original Papyrus' laugh is replaced with "Muah-ha-ha-ha!" and "Hmph." He dies at the hands of Undyne on the Route of Betrayal.

Has a cat named Doomfanger. The cat scratched at Papyrus's door after smelling his food.

Undyne
She wears a black T-shirt with a red, melted heart symbol and pants with a belt, as well as red gloves and red or black boots. Her lips are painted with a special lipstick, a gift from Alphys. Her scales have become a darker color.

She remains the captain of the Royal Guard and only appears when things get serious. She's known to enjoy sparring with Papyrus. During one of these sparring matches, she lost, and Papyrus made himself a belt. She doesn't lose her temper easily and doesn't rush into battle without thinking, but once engaged, she won't run away. She's almost as ruthless as Asgore.

Napstablook
He hates anyone associated with Alphys and Asgore. He's confident enough to tell you the truth or his opinion. He goes too far when it comes to expressing his emotions. He's not nervous, just very awkward and often laughs through awkward silences. He's not angry, just passive towards Frisk. He also doesn't believe Frisk will survive.

Sans
He differs from the canon version in that he has sharp teeth, one of which is gold. His bones are not completely white, but have a gray tint. He prefers mustard, not ketchup. He wears a black jacket with fur on the hood, tufts of which for some reason levitate, a red sweater or t-shirt, black shorts with yellow stripes, yellow socks, and red sneakers with slits. He wears gold rings on his fingers (he never wears less than five rings) and a gold chain around his neck. Also, unlike in canon, his eye glows red (in normal mode) and orange/yellow (in battle), rather than blue/yellow. His eye glows constantly, and during battle with him, the glow intensifies, and red smoke begins to come out of his mouth. He likes puns much less than the original Sans, preferring "knock-knock" jokes. He is very tired and also sweats when angry. He sells chimichangas instead of hot dogs. When meeting the protagonist, he offers his hand, but with a rubber horn instead of the classic pillow joke. He calls his brother "Boss", "Chief", and any other synonyms for the word "Chief", but he does this not out of respect or fear, as the fandom usually portrays it, but simply to joke around with Papyrus. He also loves Papyrus (like a brother). Despite his characteristics, he is very strong in battle. Instead of saying "you're going to have a bad time", Fell!Sans instead says "I'm going to have ONE HELL OF A TIME!"[15]. He is still the final boss on the Route of Betrayal, and uses gold rings as collars for his blasters and a gold chain as a chain for his collars.

Frisk
He wears a blue, pink, and black sweater and brown shorts. Frisk carries Flowey in a pot and always holds him in his arms. It's unknown how he managed to plant Flowey in a pot. He is a pacifist character in a world of monsters based on murder. He tries to befriend characters who want to kill him. He refuses to stop trying to make everyone better. He doesn't want to kill, but he's perfectly capable of hurting someone. He's very tired and always has sleepy eyes, but he's still just as determined as his original counterpart.

Toriel
She cares for Frisk, but shows it in a terrible, sometimes even cruel, manner, but will protect you at all costs. She wears a worn, torn black dress with a white Delta rune on the chest. She has more pronounced fangs, yellow eyeballs, and red bags under her eyes. Unlike in canon, she can't cook well, so she uses demonic substitutes for ingredients, such as blood, monster dust, and so on. Instead of cinnamon butterscotch pie or snail pie, Toriel gives Frisk meat pie. She asks them a lot of questions in a psychotic, confused, and erratic manner and won't look anywhere else while Frisk is looking at her. She has a very beautiful rose garden. She hid in the Ruins after learning that Asgore poisoned Chara.

Flowey
The flower that greets Frisk at the beginning of the Dungeon. Its petals are tattered. Alpha Flowey appears friendlier and kinder. He's afraid of almost all monsters, is horribly beaten, and is nearly dying, but he still remains happy, encouraging Frisk. Flowey loves to sing, but sometimes he oversteps his bounds and sings too loudly. On the Path of Betrayal, Flowey greatly deceives all the monsters, causing them to kill each other.

Underfell is one of the most famous AUs that answers the question: "What would the characters do if the situation was hopeless?"
The monsters in Underfell started out the same way as in Undertale. They weren't created by "hatred and fear," but rather, they became "hatred and fear" after being driven into the Underground. It all started with Asgore, who slowly dragged all the monsters down with him. Toriel tried to make him reconsider his decisions and his point of view, but he remained steadfast. This continued until Asriel's death, after which she began to develop a similar mindset, only instead of hating people, she hated monsters.
Throughout history, monsters eventually came to blows because Frisk interfered with their static thinking. Humans would fight, but they could never kill—only monsters could do that to each other. Frisk would try to teach them to trust humans and other monsters and ultimately help them regain their "humanity."

Flowey
Sans hates Flowey because of his weakness.

Papyrus
They have a brotherly relationship, so to speak. Sans calls him "boss" not because he's afraid of him, but to make fun of him. In the past, Asgore wanted to execute Sans for insulting him, but Papyrus intervened and stopped him. Even though Asgore could have killed Papyrus, Papyrus left a scar on him and recruited him into the royal guard. Sans was about to attack Asgore, but Papyrus stopped him.

Capabilities
Sans is slightly more powerful than his UnderTale counterpart. He can manipulate bones and telekinetically transport living souls. He can also manipulate Gaster Blasters. He can teleport, just like Sans from UnderTale. Bones and the Gaster Blasters he summons

Sans has his limits when it comes to cruelty, as he can snap at Papyrus if he goes too far, sometimes to Papyrus's surprise (things can get so bad that the brothers even fight, albeit not to the death). He's still quite humorous, though he dislikes puns, preferring knock-knock jokes. Sans sometimes calls Papyrus "bro" instead of "boss" in public to irritate him. However, he doesn't do this often, as it's a smart move.

Instead of Flowey claiming that the rule in the Underdark is "Kill or be killed," Sans is the one enforcing it. If Frisk takes the pacifist path, he'll be confused and disappointed by their kindness. Once they reach the hallway, Sans will fight them until they manage to reform him. Afterward, Sans declares he won't harm Frisk again and leaves.

(Not canon: in some fan art he is depicted with fluff on his sleeves and a red collar, as well as a collar on a chain).

The main antagonist of the Underfell AU and Papyrus' brother. He is similar in appearance to Sans from UnderTale. Sans is a skeleton, presumably born in Snowdin, who is bossed around by his younger (though significantly larger and more resilient) brother, Papyrus, who calls him "boss." He is verbally abused by Papyrus and is forced to obey his brother's commands. Some differences between Underfell Sans include his eye glowing red and his destructive nature, unlike his UnderTale counterpart.
Fell Sans is very similar in appearance to his UnderTale counterpart. He is a skeleton with sharp teeth, and his upper left canine tooth is gold (definition of canine: a pointed tooth between the incisors and premolars). He is slightly taller than UT Sans. Fell Sans wears black sweatpants like UT Sans. However, they have a vertical yellow stripe down each leg. He wears red and white sneakers with yellow socks. He has bags under his eyes (due to lack of sleep). His left eye glows red 24/7. He wears a red sweater, unlike UT Sans's white t-shirt. He wears a heavy black coat with a gold zipper. On the hood, he has white, brown, yellow, and pink (a combination of red and white) fluff. Fell Sans often sweats because his large coat makes him hot (black clothing can attract heat). He occasionally has a cracked skull (most likely due to his brother, Papyrus). He wears black fingerless gloves or red mittens, or no handwear at all. Personality
Unlike the classic Sans, who is kind and calm, Underfell Sans is a different story. He's rude, mean, and has a strange relationship with his brother. Papyrus often insults him, to the point of publicly calling him "boss." Due to his aggressive nature, Sans also displays hatred for humanity after they sealed the monsters underground.
Despite Papyrus often bullying him, the brothers still care for each other, even if they don't show it.