Names

At the beginning of Undertale, the player is given the choice of naming the fallen human. 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. 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 of these names, some (mainly those of major characters in the story) cannot actually 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.

Names disallowed (not case sensitive)

 * Sans - "nope."
 * Toriel - "I think you should choose 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 before you can confirm the name.]

Names with custom responses (not case sensitive)

 * Frisk - "Warning: This name will make your life hell. Proceed anyway?" [Activates Hard mode.]
 * Murder/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/Blooky - "............ (They're powerless to stop you.)"
 * Bpants - "You are really scraping the bottom of the barrel."
 * Gerson - "Wa 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.

Changing your name within the system file:
If you decide that you don't like your name within the run you are doing and want to reset it you can. It will also give you an extra secret in the Stats screen unless it's a few special names. If you choose a normally forbidden name appears to have no effect on the game. For example, naming yourself Sans allows you to play through normally. It also appears that you can use this to increase the name size, so you can fully become Papyrus.
 * 1) Go into C:\Users\YourName\AppData\Local\UNDERTALE within Windows (~/Library/Application Support/com.tobyfox.undertale/ For Mac)
 * 2) Then open the .Ini file named "undertale" with Notepad or Microsoft Word (any other programs similar to Notepad should work.)
 * 3) There you should see a line of texts, find the line that has: Name="Player Name" and change your name.
 * 4) You will also have to go into every Undertale Save file that appears in the folder with the Undertale.ini file and change your name as well to make sure that it sees your name change.

After fully renaming yourself and not using one of the Forbidden names you will see the text "Easy to change, huh?" near your name after you open the Stats menu.