Frisk is the playable character and main protagonist of Undertale. After Frisk falls into the Underground, they embark on a journey to return to the surface. Frisk is the last of the eight humans to fall into the Underground after traveling to Mount Ebott. Frisk's name is only revealed in the True Pacifist Route.

­ Flowey is the first character the player is introduced to. Flowey usually appears bright and cheery, but in actuality he is cruel and heartless. Flowey’s main philosophy is “Kill or be killed”, and judges the player throughout the game based on the actions they have taken, namely the monsters they have killed or spared. Flowey lacks a soul and thus is unable to feel love or any other form of emotion. Flowey is also the only other character, aside from the player, that fully retains their memories regardless of when you save and load, and is able to make reference to events that you may have saved over

Toriel is the second character the protagonist encounters in Undertale. She saves the protagonist from Flowey and guides them through the Ruins.

Sans is the brother of Papyrus and is a important character in Undertale. He first appears in Snowdin Forest after the you exits the Ruins. He serves as a supporting character in the Neutral and True Pacifist routes, and as the final boss and heroic antagonist in the Genocide Route.

Papyrus is the brother of Sans and is also an important character in Undertale. His main motive is to capture a human so he can become a member of the Royal Guard, and finally have friends and popularity.

Undyne is a fish-like monster that leads the Royal Guard. Clad in her full suit of armor, she pursues the protagonist through the entirety of Waterfall.

Mettaton is a robot with a SOUL and was built by Alphys. Initially poised as an entertainment robot turned human killing robot, Mettaton is later revealed to have only acted as such so that Alphys could rescue the protagonist, feel important and become closer to them. Mettaton is the sole television star of the Underground.

Dr. Alphys is a female, reptilian monster that lives in the lab in western Hotland. She is the incumbent Royal Scientist, a position she was awarded by Asgore after the disappearance of her predecessor.

Asgore Dreemurr is the ruler of the Underground and its denizens, monsters. He is the ex-husband of Toriel, father of Asriel Dreemurr, adoptive father of the first human, and the penultimate boss of the Neutral Route. While not evil or malicious, he serves as one of the main antagonists, planning to kill seven humans and use their SOULs to break the barrier which traps the monsters in the Underground.