>>717654716
Radagon probably comes from two places.
First, is that the Giants cursed Marika somehow, and that was symbolized by red hair. Marika seems to have passed that off onto Radagon as a splinter of her other half, leaving herself uncursed (at least in appearance) but him marked by it.
Also she was likely already having doubts about the Golden Order at the time. Because she was already scheming with Godfrey to send him off to get stronger and die so that one day she could revive him, and he'd return to reclaim his place next to her. This came about after she seemed to have been pressured to banish him by the Two Fingers.
Radagon seems to have been made of all the parts of her that doubted her path of betrayal. Instead, he was loyal to the Order and wanted to stay aligned with it. He believed in the system and wanted to study its deepest parts to learn as much as he can, as both scholar and priest.
So he was both an outlet for a curse, and a way for her to discard her fears and doubts. Eventually he developed his own true ego separate from her own. So they could never truly recombine again.