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Welcome to DETERMINATION - An Undertale for Smash Campaign! We got our Sans Mii Costume, now let's go grab that smash invite! (Thread) 
UNDERTALE is a very unique game, a RPG where you don't need to fight to achieve your goal. Instead, you can be friends with your enemies! Talk to them, dance with them... DATE THEM! (Or you can slaughter everyone without any remorse)+
The game was a hit in 2015 (Released Sept 15). by early 2016, Undertale already surpassed one million sales. Undertale turned out to be one of the most memorable indie games from the 2010s, along with Shovel Knight, FNaF, Hollow Knight, Cuphead, Celeste, etc.
Undertale was created by Toby Fox and a small group of people helping him, including Temmie Chang. Games like Earthbound, Mario & Luigi Series and Touhou Project were big inspirations for Toby. He even met with Shigesato Itoi (2016) and ZUN (2018)!+
Toby first started making music for Homestuck, and later started making some Earthbound ROM Hacks. During Undertale's development he was also working on Deltarune.
The choices and the characters is what makes Undertale so special. The game makes you care about the characters like they are real people! Also, the OST is extraordinary. A lot of them are the same music but rearranged, and they still manage to be incredible.
On September 4th 2019, Sans was announced for Smash as a Mii Fighter Costume (I can't explain how happy I was), the internet EXPLODED because of the funny man skeleton.+
Toby Fox visited Sakurai and they even played Smash Bros, and Sakurai apparently lost LOL (Makes sense, Toby was in the Melee competitive scene). Plus, he drove Toby home while they were listening to Smash music, how awesome is that?
Toby Fox even made music for Little Town Hero and Pokémon Sword and Shield (the song S L A P S)!
Undertale has a lot of characters, all of them are fun and unique. When talking about Super Smash Brothers, most people think of Sans as the UT rep, since he's the most popular character in the game.+
But Nintendo not always go for the popular character (remember that all the characters in ARMS being the protag is one of the reasons Min Min was chosen), so I think they would go with the protagonist, Frisk. But they can easily go with other characters like Papyrus, Undyne, etc.
The Human (aka Frisk) is the protagonist of Undertale, the human child that falls into the underground. Frisk would probably be one of the most interesting fighters in the game. In Undertale, you can decide between 3 routes: Pacifist, Neutral or Genocide.+
This could be implemented in Frisk's moveset, where they would have 3 different movesets based on the routes. Pacifist Frisk could fight like Isabelle, using silly things from the game or attacking by accident. Genocide Frisk would be a beast, cutting everything they see.
While Sans isn't the protagonist, he's the most popular character in Undertale. He could use all of his fight attacks + things like his trombone. I don't think it fits that much with his character, since he only fights for a motive. But it would be awesome to see him in Smash!
THE GREAT PAPYRUS, his dream of being acknowledged by society would finally come true. Papyrus would be a really fun fighter, his blue attacks would really come in handy, making Papyrus very unique and different from the others. Also, imagine alts like the "Cool Dude" outfit!
Undyne is a warrior, she was born for this! Her magic spears and her terrible cooking abilities would make her the best fighter she can be. Also, she could even have a meter that transforms her into Undyne The Undying (also suplex everyone just because she can).
OHHH YES! Mettaton, the star of the underground in smash would be really funny ngl. He could transform between Mettaton BOX and Mettaton EX (NEO is useless, unfortunately). Imagine Mettaton dancing with Bayonetta in his trailer!
Flowey would be a really interesting fighter. We don't see him fighting that much, only using his friendliness pellets. But if he turns into Asriel... He would be awesome! His final smash could even be Omega Flowey with all the souls!+
The thing is, Smash doesn't care that much about spoilers, but Flowey being Asriel is such a big thing in Undertale, I don't think it would be the best choice for an UT rep.
Not an UT fighter, but omg they would be so awesome! They could work like Pokémon Trainer/Ice Climbers, or even just Kris with the others helping him with attacks, like in RoA. Imagine if his Up B is Susie throwing Ralsei kinda like Ice Climbers, but Ralsei is panicking.
There is a lot of other Undertale characters, but I think these guys would be the ones that would work. The only indie representation in Smash is: Shantae, Shovel Knight, Cuphead and Sans. None of them as fighters, so seeing an indie character as one would be pretty sweet.+
So let's keep hoping that we can see one Undertale character join the roster, always STAY DETERMINED (We accept a spirit event too 
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I recommend that you follow @/PapyrusForSSBU and @/FriskSmash too! (Not related but also @/Craft_SSBU, Steve is 
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