[[WMG: Rose Madder is Mordred Deschain.]]
Or rather a Twinner to him, on some other level of Franchise/TheDarkTower.
** Can a being like Mordred have a twinner though? There seems to be only one Randall Flagg after all. Also, Rose Madder seems to be on the 'Cosmic Order' side of the alignment spectrum.

[[WMG: Rose Madder is a creature similar to IT.]]
Both tend toward arachnid forms when not needing a less alarming disguise and both have true forms that drive a human viewer insane.
** Which has interesting implications itself, since Rose Madder seems to be more or less on the light side cosmically speaking.

[[WMG: Rose Madder is a good(ish) counterpart of the Crimson King.]]
To put it another way, she's paired with the Rose like he's paired with the Tower.

They're both cosmic beings that wear red and get involved in the lives of people in an abusive relationship. Whereas the Crimson King puppets an abuser, Rose Madder saves the abused.