So, here's a crazy lore theory from me: Rosaria is actually a malfunctioning illusion of Gwynevere
Here's how I think it goes down:
spoiler:- the original Gwynevere leaves to marry Flann
- Gwyndolin sets up the illusion too keep Anor Londo going
- the illusion eventually gets married off to Oceiros
- somehow, Oceiros is able to father children with the illusion, all naturally imperfect because duh
- this puts a huge strain on Gwyndolin
- the Gwynevere illusion becomes impossible to maintain, and, after giving birth to Ocelotte, devolves into Rosaria
- the queen "disappears" to avoid a scandal, Oceiros goes full bonkers and is deposed
- Rosaria is actually imprisoned in the Cathedral of the Deep, Sulyvahn figures out what's happened
- Gwyndolin is now ill and weakened enough for Sulyvahn to imprison
- Gwyndolin is fed to Aldrich
- Rosaria remains, her soul a faint, distorted thing