Chara Dremurr (As we don't know their real surname, it adopted it's adoptive family surname) is the representation of the player inside the game. It was the first human to ever fall in the Underground and the adoptive child of the Royal Dreemurr Family. It's only revealed at the end of the Genocide Route as we reach the end of it. It's constantly mentioned by Flowey during the multiple Routes and we don't learn it's story until we get to the Dreemurr residence in the Capital.