undertale’s major twist at the end is finding out that the character you named isnt the character youre playing as and is their own person, while the defining moment of deltarune is the character literally ripping out the soul youre controlling them with and throwing it in a cage where you can briefly move it around establishing that you are an entity separate then them. so i think its pretty fair and obvious to say that no, these are not intended to be blank slates for you to project gender/age/etc on, thats like kind of a fundamental theme of both games here