Names

At the beginning of Undertale, the player is given the choice of naming the fallen human. While the narrative flow and naming screen is seemingly the player naming the protagonist, it is to name the first human who fell into the Underground.
Normally, the game asks "Is this name correct?" when asking for confirmation of the name. However, some names trigger a unique response, can have a different impact on gameplay, and certain major character names cannot be used. There is a six-character limit, but this can be bypassed using the renaming process as seen below. 
Once confirmed, the name is visible on the Start Menu, the SAVE file from SAVE Points, most encounters, the overworld menu, and looking at Stats. Additionally, this name is mentioned by Flowey, Asriel, Asgore in the game over screen, the dream sequences in Toriel's Home and Waterfall, the mirrors in Toriel's and Asgore's Home and the Lab console on the Genocide Route, the VHS Tapes in the True Lab, and the first human's coffin. Notably, none of the characters speak the protagonist's name until they learn about it in the True Pacifist Route epilogue.
All names (except "Frisk") cannot be changed without performing a True Reset or completing a Genocide Route. On the Start Menu, the message "A name has already been chosen" appears when "Reset" is selected. "Frisk" is the only exception, as changing the name is required to leave Hard Mode.
== Names with custom response ==
Below are listed names that give a custom response, replacing the default one. Alphabetic characters are case-insensitive and kana are script-insensitive. When played in Japanese, due to being able to enter both kana and alphabetic characters, the same response occurs regardless of whether the English or Japanese name was entered.
=== Forbidden ===
English 	Japanese 	Response
(Blank name)	— 	"You must choose a name."
Alphys 	アルフィー 	"D-don't do that."
Asgore 	アズゴア 	"You cannot."
Asriel 	アズリエル 	"..."
Flowey 	フラウィ 	"I already CHOSE that name."
Sans 	サンズ 	"nope."
Toriel 	トリエル 	"I think you should think of your own name, my child."
Undyne 	アンダイン 	"Get your OWN name!"
=== Allowed ===
English	Japanese 	Response
Frisk 	フリスク 	"WARNING: This name will make your life hell. Proceed anyway?"
Murder 	ころす 	"That's a little on-the-nose, isn't it...?"
Mercy 	みのがす 	"That's a little on-the-nose, isn't it...?"
AAAAAA 	ああああああ 	"Not very creative...?"
Aaron 	アーロン 	"Is this name correct? ;)"
Alphy 	アルっち 	"Uh.... OK?"
Catty 	キャッティ 	"Bratty! Bratty! That's MY name!"
Bratty 	アリゲッティ 	"Like, OK I guess."
Bpants 	バガパン 	"You are really scraping the bottom of the barrel."
Chara 	キャラ 	"The true name."
Gerson 	ガーソン 	"Wah ha ha! Why not?"
Jerry 	ジェリー 	"Jerry."
Metta 	メタトン 	"OOOOH!!! ARE YOU PROMOTING MY BRAND?"
Mett 	メタ 	"OOOOH!!! ARE YOU PROMOTING MY BRAND?"
Mtt 	メタ 	"OOOOH!!! ARE YOU PROMOTING MY BRAND?"
Napsta 	ナプスタ 	"............ (They are powerless to stop you.)"
Blooky 	ブルっち 	"............ (They are powerless to stop you.)"
Papyru 	パピルス 	"I'LL ALLOW IT!!!!"
Shyren 	シャイレーン 	"...?"
Temmie 	テミー 	"hOI!"
Woshua 	ウォッシュア 	"Clean name."
== Names with other effects ==
* Drak, Gigi, or Gugu — The Steak in the Shape of Mettaton's Face shows up as "FSteak" instead of "FaceSteak" (the name does not change in v1.01). This name is a reference to Gigidigi's Persona 4 comic, "Hiimdaisy". Notably, Gigi has also helped work on the game.
* Gaster or ガスター — The game reloads to the prologue cutscenes as soon as the last character of "Gaster" is entered.
* Frisk or フリスク — In addition to having a custom response, this name activates Hard Mode.
== Changing the name in the SAVE files ==
Normally, the name cannot be changed except after a True Reset or after a Genocide Route. However, it is possible to change the name during gameplay by modifying the game data. The following instructions are only applicable to PC versions.
# Open the app data directory for Undertale. On Windows, this should be %LOCALAPPDATA%\UNDERTALE (typically resolving to C:\Users\USERNAME\AppData\Local\UNDERTALE, where USERNAME is the name of the Windows user). On macOS, this should be ~/Library/Application Support/com.tobyfox.undertale/, and on Linux it should be ~/.config/UNDERTALE/.
# Open the file named file0 in a text editor.
# The name is stored on the first line—replace it with a new name.
Numbers and symbols can be added to this new name. If it is longer than six characters, it covers surrounding text, and the text "Easy to change, huh?" appears in the Stats menu. Using a forbidden name has no effect.
The name is also stored in undertale.ini, which is where the name displayed when loading a SAVE file is retrieved from; however, it is overwritten with the name from file0 when the game is saved (if they differ). If the game is reset instead of loading a SAVE, the undertale.ini name overwrites the file0 name instead.
== Trivia ==
* The Stat menu has the name written with quotation marks around it, implying that it is an assumed name for the protagonist.
* The name is absent during the following encounters: the first encounter with Flowey, the story expositions from the Monsters and Flowey in New Home, and versus Photoshop Flowey.
* All forbidden names can be used in the Undertale Demo.
* If the protagonist's name is changed to "Gaster" through the SAVE files, the ability to reset is unavailable as the game restarts upon clicking the Reset button.
* In PC versions until v1.05, the game reloads the title sequence after pressing the confirm button.
* In the PlayStation 4/Vita, and Nintendo Switch versions until v1.03-4, the game crashed and displayed an error message as the last character of "Gaster" is entered."Fixed an issue where the game crashed if you put your name in as a certain character." Undertale FAQ, Patch Notes
== References ==
de:Namen
es:Selección de nombre
fr:Noms
ja:名前のイースターエッグ一覧
pl:Imiona
pt-br:Nomes
ru:Имена
zh:名字