Names

At the beginning of Undertale, the player is given the choice of naming the fallen human.

Normally, the game will ask "Is this name correct?" when asking for confirmation of the name, but some names will trigger a unique response, and some of these names, (mainly those of major characters in the story) cannot be used. Other names will not trigger any particular message but have a different impact on the gameplay. There is a six-character limit, but this can be bypassed using the renaming process as seen below.

Although this is depicted as the player naming the main character, it is, in fact, naming the first human to have fallen into the Underground. Notably, none of the characters aside from Flowey/Asriel, Asgore in the game over screen, and the first human's coffin ever use the name of the fallen human, and never speak the main character's name until they learn it in the True Pacifist Ending.

All names (except "Frisk") cannot be changed without performing a True Reset or completing a Genocide Route. Otherwise, when resetting, the message "A name has already been chosen" will appear and the player is unable to change the name. "Frisk" is the only exception, as changing the name is required to leave Hard Mode.

Forbidden names (not case sensitive)

 * Sans - "nope."
 * Toriel - "I think you should think of your own name, my child."
 * Asriel - "..."
 * Asgore - "You cannot."
 * Flowey - "I already CHOSE that name."
 * Undyne - "Get your OWN name!"
 * Alphys - "D-don't do that."
 * Gaster - [Reloads the intro sequence without offering a confirmation prompt.]

Names with custom responses (not case sensitive)

 * Frisk - "Warning: This name will make your life hell. Proceed anyway?" (activates Hard Mode)
 * Murder or Mercy - "That's a little on-the-nose, isn't it...?"
 * Catty - "Bratty! Bratty! That's MY name!"
 * Bratty - "Like, OK I guess."
 * Temmie - "hOI!"
 * Aaron - "Is this name correct? ; )"
 * Woshua - "Clean name."
 * Chara - "The true name."
 * AAAAAA - "Not very creative...?"
 * Metta/Mett - "OOOOH!!! ARE YOU PROMOTING MY BRAND?"
 * Jerry - "Jerry."
 * Papyru - "I'LL ALLOW IT!!!!"
 * Alphy - "Uh.... OK?"
 * Napsta or Blooky - "............ (They're powerless to stop you.)"
 * Bpants - "You are really scraping the bottom of the barrel."
 * Gerson - "Wah ha ha! Why not?"
 * Shyren - "...?"

Names with other effects on the game

 * Drak/Gigi/Gugu - The Steak in the Shape of Mettaton's Face shows as Fsteak in the inventory instead of Face Steak, referencing Gidigidi's Persona 4 Comic, who also worked on the game.

Changing the name in the save files
Normally, the name cannot be changed except after a True Reset or giving your soul to Chara. However, it is possible to change the name during gameplay by modifying the game data.
 * 1) Go into C:\Users\YourName\AppData\Local\UNDERTALE within Windows (~/Library/Application Support/com.tobyfox.undertale/ for Mac).
 * 2) Open the file named "file0" with a text editor of choice.
 * 3) The name is stored on the first line. Replace it with a new name. Numbers and symbols can be added to this new name, but if it is longer than six characters, it will cover surrounding text. Using a forbidden name has no effect.

The name displayed on the main menu is stored in "undertale.ini" in the same folder, but it will be overwritten with the name in "file0" when you save. If you attempt to reset while the names are different, the name in "undertale.ini" will overwrite the name in "file0" instead.

If the game detects that the name has been edited (for example, if it is longer than six characters), the text "Easy to change, huh?" appears in the Stats menu. Oddly enough, this does not occur if a forbidden name is used.