Since I took a break from U.K., I decided to look at what I've got so far and maybe fix some mistakes, but either way, some extra info on the main cast (SPOILERS AHEAD).
Frisk: The protagonist of the story. Even after being killed, they still came back, and now try to figure out what happened, and try to get back to humanity.
Flowey: A suspicious looking buttercup. Pretty much maintains his old personality, although, something isn't clearly the same.
Hayde: A member of the Royal Guard. Despite being in san's position, Hayde is far less lazier, and actually (try at least to) do their job. In short they're far from san's personality.
Papyrus: One of the most important members of the Royal Guard. After Undyne couldn't function in the Royal Guard, he ended up taking her place. However, if something happens that even Papyrus can't figure out, he can always ask Undyne if he can't figure it out on his own. Or he just comes to visit her. He often constructs puzzles with Hayde if they need some extra hands. I could ramble about papyrus for an eternity, but there's just way too much to cover.
Alphys: The Royal Scientist. Despite the Amalgamates having been discovered, Alphys managed to keep her job due to the situation they've been put in after the barrier was destroyed. She has helped with building the city (aka Hotland/core) and some great other stuff. But, there's still something laying deep beneath Alphy's lab...
Undyne: The former Royal Guard Leader. Due to health conditions after barely surviving in her undying form (which can be seen on her ears) she couldn't function properly as a leader anymore. However, she still gives sport training to whoever wants, and assists Papyrus with his job. I also could talk about undyne, but I'll come back to her an other day.
Mettaton: He's the star of monsterkind. He still has his TV show which he runs with Napstablook and Shyren. After being heavily damaged on the genocide run (not killed) Alphys managed to save him, and give some upgrades to him. He also asked the dummies for assistance, so we can say he reunited with his family.
Toriel: The queen of monsterkind. After the barrier was destroyed, Asgore left the throne to her after somewhat calming the whole situation they were put in. Toriel is still making her pies, but struggles to maintain any sort of plants, which explains the lack of them in her house and throne room. She has complex feelings regarding Asgore.
Asgore: After leaving the throne he takes care of the graveyard in the former throne room. He also takes care of surface home, a place where monsters that we're to terrified to go too far from the mountain live. Some even still live underground. Asgore is capable of maintaining flowers and plants in a healthy condition, but struggles on cooking (he's still improving).
Muffet: A spider that runs a successful bakery. Thanks to advice from a certain person with a hidden face, she ended up lowering the prices, but ended up selling more of her food, yet get the same amount of money. However, she can still get quite greedy.
Mystery man: an unknown being, trapped in the void where no one will remember you (except some followers who are referring to Gaster, and a few monsters. Nobody actually confirmed that this thing we call Gaster is actually Gaster, he could be someone else.
Chara and Asriel: the two dead children of Asgore and Toriel. Chara and Asriel gave somewhat changed their appearance, but their roles haven't changed much. Chara (my headcanon) reawoke as a narrator, while Asriel is still flowey.
Sans: He's dead. That's it. Little to no people know the true reason why his life came to an abrupt end. And his fate is undetermined.
