Frisk - One of the main protagonists. Frisk is a volunteered assistant for Sans' studies on timelines, because of their knowledge on being able to reset timelines. Compared to their Undertale counterpart, Frisk has little to no differences in both personality and looks, apart from wearing dark blue pants, brown boots, having a few hairs sticking up, and a red heart locket. This heart locket, however, does not link to the Golden Locket at New Home. The reason why Frisk was interested in helping Sans, is that they believe it would've been generous if Sans had company, and that Science seems to interest them.
Sans - One of the main protagonists. Sans was one of the assistants that helped the previous royal scientist Dr. W.D. Gaster build the CORE. After the royal scientist's death, Sans starts to experiment and study on timelines. To get physical data, Sans, along with Frisk's help, constructed on a Timeline Machine, which can travel them to different timelines. Compared to his Undertale counterpart, Sans is more open to his interest in science, though doesn't talks about timelines and such to his peers, due to their unawareness of them. Sans seems to be lazy in the outside world, but when he's on his research, Sans doesn't slack around and more focuses on his work. Sans takes a lot of enjoyment on humor, and always gets a kick out of cracking a pun or playing a prank. Sans wears a light blue lab coat, dark grey sweatpants, a yellow bow-tie, and unlaced blue sneakers with white toes.
Flowey - One of the main antagonists. Flowey finds it entertaining as how Frisk and Sans are finding research to different timelines and dimensions, and tries to find a way to steal the six human souls from Asgore, to use it as a power source for the Timeline Machine, since the only way the Timeline Machine can be activated is through Sans's magic. By doing this, he would be able to travel to different timelines and tamper with them, and eventually destroy the entire existence of the universe. Flowey has scars near his eyes, holes in his petals, and a carved smile on each side of his cheeks.
King Asgore - The king and ruler of the Underground, and is in need of seven human souls, along with one monster soul, to reach back to the surface. Asgore keeps the souls in containers at The Barrier, and is an obstacle in Flowey's plan for the destruction of the universe.
Papyrus - The brother of Sans, and is also in training for the Royal Guard. Papyrus is completely unaware of Sans and his research, and assumes he's usually "sleeping somewhere", much to his annoyance.
Dr. W.D. Gaster - The first Royal Scientist of the Underground, and creator of the CORE. His death is presumed to be a failure in his creation, shattering him across time and space. Sans was one of his assistants, which gave Sans his experience in knowledge of timelines.
Alphys - The current Royal Scientist of the Underground.
Toriel - Caretaker of the Ruins and was the queen of the Underground.
Annoying Dog - Annoying.