Team PSFT (Pacifist): Papyrus, Sans, Frisk, and Toriel. When this popped up in my head, I knew I had to share it. Is it stupid and random? Maybe, but I really want to see it happen; preferably before I get shipped off to BMT next week.
Q: So do the monsters all have to be human?
A: Of course, Papyrus and Sans both have to be human, and Toriel a goat Faunus.
Q: Their ages?
A: Sans and Toriel would be 17 like most of the other students at beacon, and Papyrus and Frisk would be 15 like Ruby. I always thought of Papyrus as the younger brother type compared to Sans. Now speaking of siblings, I was thinking that since Toriel would be around the same age as everyone else, she would be Frisk's older sister. The backstory that I thought up would be that Frisk is an orphan who was adopted into Toriel's family. Toriel will of course be very protective of Frisk.
Q: What about their Semblance?
A: Ah, now this is the interesting part. I was thinking of them having abilities similar to the ones they have in game. Papyrus's Semblance would his ability to summon bones, Sans' Semblance is bone manipulation (ex: using his Semblance he can manipulate the bones of others and fling them around like how he uses his blue magic in the game, and also maybe his bone manipulation won't be limited to moving bones around, but maybe he can also teleport bones/living organisms with bones, so using said bone manipulation would also allow him to teleport himself and others around), and Toriel would of course have a fire Semblance. Now Frisk on the other hand, is a tricky one. I believe the only Semblance that would suit Frisk would be time manipulation that is very similar to using DETERMINATION to SAVE and LOAD in Undertale. Meaning that Frisk would use their Semblance to revive/go back in time to a point before they died, use knowledge of the future to dodge attacks, and they would of course use it to save others. Frisk would not abuse their Semblance because they know that would be wrong, also Sans would notice them using their power for their own gain, and then proceed to dunk on them.
Q: How about their weapons?
A: Papyrus will use his bones, Sans will use his Gaster Blasters (although he won't use them unless he has to), maybe Toriel will use Asgore's trident, and Frisk will usually pull out a toy knife. Even though they probably won't try to use it on anyone even though its fake.
Q: Hey! What about all the other Undertale characters?
A: I'll be honest in saying that I'm not sure. I was thinking about Asgore being Toriel and Frisk's father, Asriel being Toriel's little brother that tragically dies along with their human friend Chara. Other than that, I do not know. Hopefully whoever picks this up can figure it out because I sure can't.
That's all I have, good luck to anyone who accepts this challenge!