This question cannot be asked without giving spoilers, so please don’t step forward without truly playing it.
After completing a True Pacifist run and opening the game again
Flowey appears asking for you to leave it as is, to not reset the game, because you, “Player Name”, are the only one with the power to do that.
But that confused me a lot, because we
don’t name the playable character, but instead the first human to go underground (Chara), who manifests themselves and rules over everything erasing the world in a Genocide Run. But we aren’t Chara, because we sell our soul to them to be able to ever play again
So… who is the player really?
You play as Frisk, the purple and blue striped child of indeterminate race and gender. Frisk is the character who you play as and, unless we really want to get philosophical about things, the player is Frisk.
Chara, the fallen child, or whatever you named them, is not the player. You never control them- all you do is name them.
Naming a character who is not you is nothing unique. You name your rival in the Pokemon series, yet I’ve never seen anyone get confused and think they’re Gary Oak. The only difference is that Undertale misleads you and, in its defense, it never said whose name you were entering.