Let’s get to it. Deltarune is an Alternate Universe from the one in Undertale. There’s still lots of things potentially to uncover about what’s different but it seems like the Deltaverse reverses the biggest aspect of the characters.
SPOILERS
It’s opposite world.
Undertale didn’t happen. Monsters live on the surface.
Asriel is alive and grew up normally. Chara, with their own name, also is here and seems to have had a normal live with a loving family.
Undyne and Alphys don’t know each other, and Undyne has both eyes. Alphys is clearly pretty lonely, spending her time in an alley, feeding a cat she’s never even met.
Alphys DID however changed her opinion of Mew Mew 2, and now thinks it was good, despite her frequently stated aversion to it in the original.
Onionsan used to proclaim “The name’s Onion-san! Onion-san y’hear!” and when you find him in town in Deltarune he’s forgotten what his name even is.
Gerson, best known for being the oldest monster, and survivor of the big war, has died.
Bratty and Catty’s families live next door to each other, but aren’t friends.
Sans is the guy that ‘everyone knows’, has a whole barful of people who wait on him to show up. In the Deltaverse he’s new and knows no-one. (Except you I guess)
Papyrus – outgoing, boisterous Papyrus, who I think has the most lines in Undertale – does not even appear, and doesn’t leave his house.
Mettaton, who we only knew as the sassy robot, appears to be living in Napstablook’s house and probably doesn’t want to come out because he’s not a robot. He’s a ghost without the form he desired, because Alphys isn’t a scientist and hasn’t built it.
That’s the ones I’ve noticed so far, and the only thing that’s not reversed that could’ve been is that Toriel and Asgore are divorced. So they’re doomed in any universe I guess (but what did Asgore do here that wasn’t killing human children??)
The most obvious reason for this is that SOMETHING earlier in the timeline, before the events in Undertale, was changed. Something apparently brought down the Barrier way, way before Frisk ever showed up. The monsters are free, but none of the bonds built by Frisk during their adventure happened.
So the most obvious question that needs answering in the Deltaverse is: what broke the barrier early? Gaster is implied to have a hand in Deltarune and I remember thinking about how the DT Extraction machine was probably his design. If Gaster hadn’t been erased, had been able to continue his work instead of Alphys taking over, perhaps he might’ve been successful?
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