Undertale | ALL Undertale Naming Eastereggs!
Although this is depicted as the player naming the protagonist, it is, in fact, naming the first human to have fallen into the Underground. This name is mentioned by Flowey/Asriel, Asgore in the game over screen, the dream sequences in Toriels Home and Waterfall, the mirrors in Toriels and Asgores homes on the Genocide Route, Alphyss console on the Genocide Route, the video recordings in the True Lab, and the first humans coffin. Notably, none of the characters ever speak the protagonists name until they learn about it in the True Pacifist Route ending.
Normally, the game asks “Is this name correct?” when asking for confirmation of the name, but some names trigger a unique response, and some of these names (mainly those of major characters in the story) cannot be used. Other names do 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.
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” appears and the player is unable to change the name. “Frisk” is the only exception, as changing the name is required to leave Hard Mode.