Clover is based on Dess. That’s why she likes Ralsei so much and is implied to be a dyke for Susie, the dragon girl. Scrap dialogue has the lesbian head refer to Susie as “the girl”.
This means there are three Desses.
Meaning the lost girl and the knight are not the complete package.

With the Knight being a heavily fragmented being, maybe Papyrus was the one to put her together and give her a costume. Since the forgetton man, Papyrus, gives you an EGG [Knight movement in Wingdings] and the forgotten man speaks in chapter 3. 3 is Papyrus’ number in Undertale.

My guess about the final boss is that it’s going to be the third Dess formed and completed by the offerings of the Knight, the lost girl, Undyne, snd Toriel. And you break off the Knight, the lost girl, Undyne, and Toriel of her with Papyrus, Asriel, Alphys, and Sans.
You have to seperate their wills by who they are the most attached to.