Chara stays as Chara (in most ways).

Frisk becomes the Unused Human, however their SOUL is made from a mix of Asriel and Frisk's SOUL's.

Flowey's role is taken by a Puppet version of Frisk (Possibly a mime too?), however, Flowey himself is now your little helper! following and helping you anywhere he can!

There isn't really anyone in the role of Toriel, however, Toriel is a shop owner!

Sans stays as Sans, however, he has joined the Royal Guard as a Park Ranger!

Papyrus stays as Papyrus, he is the assistant to MTT, and has been given his own division of MTT Industries!

Undyne stays as Undyne, and is the main overseer of the underground, and is the one who would replace Gaster in his absence.

Alphys swaps roles with MTT, and is now a CrAaAaZy magician and gameshow host, her show(s) are produced and funded by MTT Industries!

MTT swaps roles with Alphys, they are the founder and CEO of MTT Industries, the biggest company in the Underground!

Asgore swaps with Gaster, Asgore is a shop owner, just like his Ex(?)-Wife, Toriel!

Gaster swaps with Asgore, Gaster is the Emperor of the Underground, he is trying to be more active and involved with the people of the Underground!
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Please keep in mind that these are not all final, also, if you have any suggestions for characters, areas, items, mechanics, etc, or any feedback, don't be afraid to say em' in the comments!