When naming the fallen human, you can type in the names of characters in the game to get different results.

(Only six letters allowed)

Names that are not allowed:
Flowey: I already CHOSE that name.
Toriel: I think you should think of your own name, my child.
Sans: nope.
Undyne: Get your OWN name!
Alphys: D-don't do that.
Asgore: You cannot.
Asrial: ...

Names that are allowed:
Papyru: I'LL ALLOW IT!!!!
Chara: The true name.
Gaster: *takes you back to intro sequence*
Temmie: hOI!
Metta (or) Mett (or) MTT: OOOOH!!! ARE YOU PROMOTING MY BRAND?
Nabsta (or) Blooky: ............(They're powerless to stop you.)
BPants: Your really are scraping the bottom of the barrel.
Catty: Bratty! Bratty! That's MY name!
Bratty: Like, OK I guess.
Aaron: Is this name correct? ;)
Woshua: Clean name.
Jerry: Jerry.
Shyren: ...?
Alphy: Uh....OK?
AAAAAA: Not very creative...?
Gerson: Wah ha ha! Why not?
Murder (or) Mercy: That's a little on-the-nose, isn't it...?

To initiate hard mode:
Frisk: Warning: this name will make your life hell.

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Next fact: Changing Your Name