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?