They are the main character and yes I said they because they don't have a gender from what I know some say frisk is a girl and chara is a boy but chara dose not have a body so I think they are a girl but I do not know and chara ends up killing every body in the underground so...
( chara is the killer and frisk is the befriender )
Anyways chara is the first fallen child and was adopted into the dreemerr family who cared for them like