Frisk

Frisk is the 8th human to fall into the Underground. And with 6 human SOULs already collected, you bet every monster is going to want to crown themselves as the savior of the monster race. But if Frisk somehow manages to make it through the Underground alive, they may get back to their normal life again. If that's even what they want.

Mettaton

After Napstablook got chosen as the monarch instead of Mettaton, he was so offended that he ran as far away as he could. That just so happened to be the Ruins. And there just so happens to be monsters there who have no connection to the outside world. He's the sole source of entertainment and news in the Ruins, and (almost) everyone looks up to him. He'll do anything to put on a good show.
One day, a robot came stumbling into his house and collapsed on the floor. He asked his friend Alphys to help fix it up into something more "presentable". After all, why shouldn't he have multiple forms like the human actors he adores.

Toriel

After a human tried to assassinate her and Asgore, she fled to Ruins, betrayed that her husband failed to defend her. Assumed dead by the rest of the Underground, she lives in her old home in the Ruins. She vows to protect every human that falls down, worried that if they might leave the Ruins... Their husband will surely kill them as revenge for "taking his wife's life".

Alphys

She was the Royal Scientist all the way up until she created the amalgamates. The pressure got to her, but both her assistant Papyrus, and the newly comatosed Asgore, urged her to admit the truth and give up her position as the Royal Scientist. Now she lives in Snowdin with her girlfriend Undyne and experiments with all sorts of wacky inventions. Meanwhile she also oversees the defenses of the Royal Guard's Training Grounds.

Undyne

She's the captain of the Royal Guard, and holds her post in Snowdin. She normally patrolled Waterfall, but after a harsh training accident that cost her her eye, she moved to Snowdin. It's easier there anyway, since the Royal Guard Training Grounds are only a stone toss away. One of her responsibilities as captain is to train the guard and keep them in shape. Usually this consists of marching routines and the works, since an old decree made the Royal Guard more eye-candy than an actual military force. But screw that! The monarch is too cowardly to take a life in her eyes, so she'll just have to do it herself. With Napstablook having pretty much shut themselves out from the world, the people of Snowdin look up to her as an unofficial mayor.

Asriel

After he had gone into hiding, he studied a lot of cooking. Just in case he ever were to need cooking skills again, he would never make the same mistake he did when poisoning his father. The days passed, and one day, Chara suggested that they went their separate ways. Claiming that if they were ever found, having them around would only put Asriel in more trouble. Reluctantly, Asriel agreed, and they separated. Asriel hid for what felt like a lifetime, and to many, probably was. But because neither of his parents were there to nurture him, he stayed a child. He found ways to increase his age, and steadily became more skilled at cooking. After a while he found joy in it. Later when news spread that the Royal Scientists had woken Asgore up from his comatose, he deiced it was best if he came out of hiding. He didn't make a big deal out of it. But he just appeared in Snowdin, took over the abandoned Grillby's building and renovated it into the café Hopes and Drinks.

Sans

Though joining the guard wasn't his first plan, he quickly moved up the ranks and became sergeant. Joining the guard was initially just to prove a point, but he found joy in trying to be active. Although these days, there are a lot of times where he just lies in a beach chair and calls it "holding his post". He's the sole patrol of Waterfall after Undyne had to move to Snowdin. The residents there love him, and he's more of a local comedian instead of a feared guard. Sans enjoys his life, and although he wishes he could spend more time with his brother, he makes up for it by using gadgets his brother invents for him. Papyrus normally tries to call Sans out for his laziness, but hey, he's the one making things easier.

Papyrus

Papyrus was the assistant to Alphys when she was the Royal Scientist. But when she quit, the torch passed over to him. It’s a lot of responsibility, but he tries to make the best out of it. Finally he’s gained the popularity he’s been looking for his whole life, right? Well, although he tries his best to invent all sorts of knick-knacks, there’s nothing that gets him the recognition he believes he truly deserves. He has made some truly impressive inventions in his time as Royal Scientist. He’s turned the CORE into a puzzle paradise and scattered minigames across Hotland. He’s created a high-tech defense system all the way from the Royal homestead in New Home to the Throne Room. And he’s helped Chara with staying hidden in the abandoned Spider Layer. Although there’s one thing he wishes he could fix above anything else. And that is Alphys’ mistake.

Asgore

After being poisoned, almost assassinated and put in a coma for a lot of years, it’s safe to say that Asgore has been through a lot. And with waking up to an unfamiliar Underground to new faces he’s never seen before, what’s the point in taking back the throne? Yes, he would probably fair better than the current monarch, after all there’s not much you have to do to beat doing absolutely nothing. But Asgore doesn’t have to be king to guide his people. He’s close to Papyrus and helps him in finding new ways to improve the lives of the Underground residents. He often visits Asriel in Snowdin with new flowers from his garden. And although it’s nice to know some people from his life before he went into a coma, he still deeply misses both Toriel and Chara.

Chara

The world hates them, so they have to hate the world, right? They’ve been the most determined human to have fallen down yet. Thus none of the humans have been able to reset when they’ve died. That’s what keeps Chara going. Knowing they can contribute something to monster society is why they’re still alive. Why they keep pushing. When monsters are given freedom, that’s when they can give in. As Papyrus has found out, since their Determination has links to resets and time warping abilities, they’ve been able to live this long because of it. If they didn’t have it, they surely would have passed away ages ago. But they’re determined to stop the next human that comes through. For Asriel. He deserves that much after what they put him through.

Napstablook

The most powerful monster in the Underground. Being monarch comes with its fair share of benefits. Too bad Napstablook doesn’t use any of them. They’re afraid they stole the throne from Mettaton, but to their knowledge, he’s disappeared. Napstablook doesn’t dare make any changes to the Underground, in fear that their people would hate them even further. And because of the intense defense system that Papyrus put up at their, no one dares tread through it, and they spend their days alone. There are claims that Napstablook has absorbed a human SOUL and took the trone by force. But that’s just legend. They only have to do one thing in their life. Acquire a single human SOUL, and shatter the barrier with the rest. That’s their duty. Even though it sounds easy… they can’t exactly say they’re prepared.