- Daveigh Chase as Frisk, a young human who climbs Mount Ebott for their own mysterious reason. It is said that those who climb the mountain never return. Frisk is also the main protaganist of the game.
- John DiMaggio as Flowey, a talking flower which was revealed as the main antagonist in the game until it turned out that it was actually Asriel who was trapped inside of Flowey.
- Kirk Thornton as Asriel, a biological child of Toriel and Asgore until his soul was turned to darkness so he turned into the secondary antagonist Flowey, later he becomes the true main antagonist in the game.
- Roger Craig Smith as Netablook, a Ghost who's the Cousin of Mettaton.
- Glenn Close as Toriel, the second character encountered in the game, she is an anthropomorphic goat who saves Frisk from Flowey, and guides them through the Ruins. She is like Frisk's mother.
- Jonathan Freeman as Sans, the older brother of Papyrus, and first appears in the forest outside Snowdin. Sans is a major character in the game. He becomes one of Frisk's first best friends.
- Tom Kenny as Papyrus, a younger of the two skeleton brothers, the elder being Sans. He is a major character in the game. He can not cook well, and his puzzles are easy. Frisk goes on a date with Papyrus in the game.
- Jim Cummings as Mad Dummy, a Living Dummy who Fight Frisk.
- Noël Wells as Undyne, A fish-like warrior who has a crush on Alphys. She is a terrible cook and once burned down her house, she is rather hard to kill hence her title of "Undyne the Undying".
- Phyllis Smith as Alphys, An independent scientist who assists Frisk if she is at a puzzle in hotland, or a fight with Mettaton.
- Scarlett Johansson as Muffet, a Spider who Makes and Sells Baked foods. But frisk don't buy muffet's bakery then She'll fight Frisk. Then a spider with a paper tells muffet to read the note and She's set Frisk Free and it's frisk spear her.
- Weird Al Yankovic as Mettaton, the only appreciated idol in the underground. Mettaton has 3 forms.
- J.K. Simmons as Asgore, The royal king. Got "divorced" with his wife, Toriel. He is one of the boss fights in Undertale. For real, he doesn't want to fight Frisk.
- Ahney Her as Chara. The first human to fall down undeground. In the game, she appears in the genocide run and a Unknown boss to Frisk to Fight for.
- Matt LeBlanc as W.D Gaster, A ghost-like creature named after the Wingdings and Aster fonts, he makes a cameo somewhere in Alphys' lab. In the game he appears as an Secret Boss.
- Vincent D'Onofrio as Grillby, The owner and fry cook of the eponymous underground restaurant.
- Temmie Chang as Temmie, The dog thing that has trouble saying things like hi, for example, she says hoi, Temmie is also a shopkeeper in Tem village.
- Tara Strong as Charlie Dreemurr, a 2nt biological Child of Asgore and Toriel and the Young brother of Asriel. He's Appeared in Pacifist and Genocide Ultimate Ending.
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