Main

Frisk

The eighth human to fall down Mt. Ebott.

Flowey

A cowardly sentient flower who initially believes that killing is the only way to survive, but realizes otherwise thanks to the human.

Adaptational Nice Guy: He only upholds his "kill or be killed" mentality because everybody else does. In truth, he's deathly afraid for his life to the point he can't work up the nerve to fight. While initially skeptical of sparing enemies, he warms up to it after getting past Toriel.
Support Party Member: He tosses Frisk green friendliness pellets as healing mid-fight.

The Ruins

Toriel

The caretaker of the ruins whose time of being isolated and stricken with grief had driven her over the edge.

Napstablook

A melancholic ghost who can be found within the ruins.

Sans

Adaptational Villainy: Oh hell yeah. Canon Sans is a comical prankster who, while capable of killing Frisk or Flowey without much trouble if he's driven to fight them, is harmless otherwise, besides admitting that without his promise to Toriel, the human would be dead where they stand. This Sans is nowhere near that, as he constantly tries to jump Frisk and kill them. The only one holding him back is Papyrus, and even then he's trying to push him away from his Honor Before Reason tactics and just let him kill the human already. His appearance in Deltatraveler reveals that he also killed an orphaned Suzy, and not only holds no regret over it but recalls it as "the most cathartic kill in [his] life."
Hypercompetent Sidekick: He seems to see Papyrus as his "boss," much to Papyrus' chagrin. At the very least, this means Papyrus' word is Sans' command... most of the time.
Mysterious Past: In Deltatraveler, he has some sort of grudge against Kris, Susie and Noelle for something their GG!Underfell selves did to him. He later claims that killing Suzy was cathartic, implying whatever she did, she stood out.

Papyrus

Adaptational Intelligence: In canon, Papyrus' puzzles usually fail due to factors he doesn't account for, like the tile puzzle just so happening to give a straight line to the exit or leaving his tracks in the snow to show the human the safe path through the electricity maze. Here he accounts for those errors, at least partly thanks to Sans, which means his puzzles are actually troublesome.
A Lighter Shade of Black: Next to Sans, Papyrus is trying to kill the human as well but he's far less dangerous due to his need for killing to be art, which means he gives them far more chances than anybody else would. In Deltatraveler, he even says that he'll give up on killing people, and instead make the Underground a better place through kinder means.
Honor Before Reason: It's his need for the human to fail his puzzles before he kills them that's kept Sans from killing them, as he only ever listens to Papyrus. Otherwise, it wouldn't be art.

"Ryno" (Chara)

The first human to fall down Mt. Ebott. They are named after the in-universe player.

Adaptational Name Change: Downplayed. Implicitly, their "canon" name is still Chara, but in the actual series, the player named them "Ryno."