There are a LOT of skeletons and I know I don’t always use the same nicknames as everybody else. Hopefully, this will help you keep them all straight.
Undertale - Sans and Papyrus
The original skeleton brothers. These are the lovable skeletons you met in the game. Well, my interpretation of them anyway. Sans is lazy and punny and Papyrus is Great - even more now that they’ve been on the surface for a while. Sans works at the planetarium. Papyrus is the Monster Ambassador.
Underswap - Blue and Stretch
Sans and Papyrus if their personalities were switched. Blue is loud, energetic, optimistic, and ambitious. Stretch is quiet, lazy, punny, and struggles to find his ambition. (It’s around here somewhere…maybe it fell into the crack between the couch cushions?) Blue works as an instructional assistant at an elementary school during the day and a security guard at a big company in the city at night. Stretch is technically a published author, although he hasn’t written anything in this timeline. He did recently get back to writing, though.
Sans and Papyrus if their world was…not exactly evil but pretty gung ho on the “kill or be killed” idea. Red is an anxious mess masquerading as a marshmallow masquerading as a tough guy. Boss is a noble soul hidden beneath way too many layers of bravado and grumpiness. They had a falling out a few years ago and only recently rekindled their brotherly bond. Red works as a service tech at a computer store. Boss is the office manager for a big company.
Swapfell - Black and Mutt
Sans and Papyrus if Underswap and Underfell had a baby. Their world was “kill or be killed” too, but their personalities are swapped. Black is a Jane Austen hero in skeleton form who prefers to keep his issues to himself. Mutt is an absolute sweetheart who is better with animals than with people. Black is a police detective. Mutt works at the animal shelter.
Horrortale - Sansy and Sweets
Sans and Papyrus if they lived through the neutral run where Undyne becomes empress and everything went really, really wrong. They did eventually get to the surface, but things got pretty dark before that happened. Sansy escaped with a hole in his skull, memory problems, and a bit of a snarky side. Sweets has braces to correct the problems with his teeth, glasses to help his eyesight, and the same sunny disposition as always. Sansy works for a package delivery service. Sweets works at Muffet’s bakery.
Mobtale - Bones and Lucky
Sans and Papyrus if they lived in a 1920s gangster version of Undertale. Monsters were never trapped underground in their timeline, but they were second class citizens. It’s taken them a while to adjust to this timeline, but they’re doing all right. Bones is still a punster with a wicked grin, but he’s a bit out of his depth at times with modern stuff. Lucky is a perfect gentleman with a strong sense of justice and a love of music. Bones is an independent hot dog vendor. Lucky is a lawyer with the Human-Monster Alliance.
Mafiafell - Smiley and Grim
Sans and Papyrus if Underfell was a 1930s gangster version of Underfell. Their timeline was rougher than the Mobtale boys’ (just like Underfell was rougher than the UT timeline) and they were involved in some shady stuff. Smiley is a big flirt with a heart of gold. Grim can seem really uptight and stern, but if he likes you, he’s the best guy you could ever have on your side. They both run a casino out in Vegas, but mostly work remotely nowadays.
Gaster Brothers - G and Aster
Sans and Papyrus if they each fused with a version of Gaster. These two aren’t technically brothers - they’re from two separate timelines - but they found each other in the Void and adopted each other. They found a way out of the Void that involved going through several other AUs before they finally reached the Best of All Timelines. G is a monumental dork trying to act like an incorrigible flirt. Aster is a shy nerd who just wants to make everybody happy. G doesn’t have a job yet. Aster is a college professor.
Outertale - Comet and Captain
Sans and Papyrus from SPAAAAACE! They followed a mysterious signal to this timeline and ended up crashing through the machine, breaking it forever. Comet is a shy boy who loves plants. Captain is a fearless and bold fellow with a flair for the dramatic and a love of animals. Comet is going to college to get a degree in botany. Captain works at the animal shelter with Mutt right now, but is thinking of looking into jobs at the aquarium instead.
Dancetale - Break and Tango
Sans and Papyrus from a universe where monsters dance instead of fighting. Every monster has a specific dance style and type of music that is their specialty. It comes from their soul. They can learn other dance types if they want. Break is generally easy-going, if a bit shy, and he doesn’t dance (anymore). His style was hip-hop/break dancing. Tango is a dramatic and flirtatious fellow who loves to dance and will encourage anyone to join him. (You can guess what kind of dance he’s best at.) These two have been in this timeline for years now but they went on tour with a dance troupe (well, Tango danced and Break was there for moral support) and only recently rejoined the group.
Underocean - Bubbles and Finn
The UO bros came through the machine back before all the boys lived at Anne’s house. They couldn’t really stay long with the main crew, so they found their way to Sweet Sands and have lived in the water near there ever since. They spent a lot of time with Blue and Stretch when those guys lived in Sweet Sands. They don’t NOT get along, but Blue and Delphin butt heads sometimes and their brothers tend to rile each other up for annoying their brothers. Bubbles is laid back and goofy and likes to hang out with his pal who runs the souvenir shack. Finn is proud and a bit of a show off. He helps his buddy the marine biologist take care of the coral reef.