Player

The player is the real-world person playing Undertale. The player is asked to enter a name to give to the Fallen Human. Toby Fox suggests that the player enter their own name.

Involvement in Story
Before starting the game, the player does not name the player character, Frisk, but in fact the first human. For the purposes of this wiki, that name is "Chara", the default name. The player is a distinct person from both Frisk and Chara, but shares the same name as Chara.

The fact that Frisk and Chara are different people is kept secret until the end of the Pacifist Route. To keep this secret, the game has nobody call Frisk by either name. The only exception is Flowey, who mistakes Frisk for Chara and refers to them as such throughout the entire game.

In a few parts of the game, certain characters will address the player directly instead of Frisk. For example, after the True Pacifist Ending Credits, Flowey tells them that they are the only remaining threat to the world with their ability to reset it, and asks them not to, so Frisk and the others can live on happily. During the ending of the Genocide Route, the player is addressed directly by Chara, who explains that it was the player's determination that allowed Chara to come back to life without a soul.