Player

The Player in Undertale controls the protagonist- Frisk. They are charged with naming the "Fallen Child," who is purported to be the playable character, but in all reality is the First Human.

Involvement in Story
Despite the fact that the player names the original human (the one befriended by Asriel), Frisk is called the name given to the original child throughout the entire game, until the ending of the Pacifist Route. This hints towards the fact that the player's decisions in many ways represent the influence that the soul of the original human had on Frisk. If the player ends up on the Genocide Route this shows that "influence of the first soul" (the player) has fully possessed the character, and taken control to try and destroy the world.