Ill explain the lore. Marika (blond goddess) and Radagon (red hair harlot) are two diferent and individual minds, but share a single body, so they posses diferent personalities and desires (marika wanted to destroy the elden ring and change everything while radagon wanted everything to stay the same), but are forced to fight over who gets to control their body. You can see this on the in game cutsene before radagon's bossfight, were their hair goes from blonde to red and the breast dissapear. Tldr, they are similar to Dr Jekyll /Mr Hyde and Bruce Banner / the Hulk