Rue is the son of six and mono, his child form comes from an alternate timeline where six never betrayed mono and they just stayed in the nowhere together trying to find a way out; this is called the "good timeline" form. However, in LN2's canon ending where six dropped mono into the flesh void and went to the maw alone: six ended up taking over the maw after she killed the lady and became an adult. As time went on, six got lonely and instead of having a biological child, she made one out of old mannequin parts and used the souls of children who died on the maw to bring him to life; this backfired and now rue is a homocidal maniac who destroys everything it sees. In this form, rue is called "the forgotten doll" and this is his bad timeline form