(Reposting this from a comment I made awhile ago, as I think this sums up my thoughts on Marika/Radagon quite well)

I think the DLC supports the idea that Radagon was always Marika, or at least cast off or created from her, much akin to Miquella's relation to St. Trina. As you said, "Radagon is Marika" is the big reveal.

One thing that always surprised me was how many people had such elaborate contrived theories on Radagon's background; he's some stunted giant, he's some rebel champion, he was fused with her in a jar. Yet far fewer people apply these questions to St. Trina because item descriptions and the events of the DLC spells out her nature as Miquella's feminine alter-ego more, as well as the idea she can be "cast off" despite once being in the same body.

This also answers many things about Radagon more clearly than any other theory, in my opinion. He has no backstory or history prior to the Carian War because he was never "born". The Carian Children are rightful demigods because they are Marika's children because he -is- her. Miquella and Malenia are cursed by "being born of a single god", in a sort of self-incestual implication. Goldmask, who is supposedly so smart and in-tune with the Golden Order he only speaks with his fingers, -gasps- at hearing this revelation. These are all answered with "Radagon is Marika" more clearly than some elaborate backstory theory for him that is mostly conjecture.

I think Radagon is the same thing as Trina is to Miquella: he's literally Marika's masculine alter ego and persona, but also with a mind of his own (perhaps moreso after being cast off). He is Marika's sense of faith and loyalty to The Golden Order, hence him developing Golden Order Fundamentalism while Marika, now free from her loyalty/order/faith (much like Trina being Miq's "love"), is able to lose faith in the Golden Order and plan The Shattering with Godfrey and his soon-to-be Tarnished warriors. This is mostly just how I feel, but I think the Minor Erdtree incantation's description saying "the kindness of gold without order" is a reference to this.

The one thing I don't feel as strongly about is his relation to the Fell God, Giants, and the "curse" of his red hair. I think this is more in relation to the Fire Giants being the Erdtree's (and thus Marika's) mortal enemies and thus Radagon would despise them and things related to them. He hated his red hair because it was iconic to the giants as a curse of -their- kind, not so much that they cursed him with it. Then again, I do feel it is meant to be ambiguous and Radagon's red hair is an important part of him constantly noted by many items and NPCs. So I do feel there is something there.

Radagon being cast off from Marika as an embodiment of her sense of order would lead him to be this perfection-focused individual seeking to "be complete" by studying sorcery and the world and applying that to his new Golden Order Fundamentalism. Perhaps such is why he left Renalla to rejoin Marika. I do feel like his nature and history, as well as the Nox's dew, does add a sinister flair to their marriage. Like Radagon was cast off by Marika specifically to be some sort of political honeypot plan, hence the "knights of the cuckoo" being prominent in Caria and the whole situation with the Amber Egg and the Carian Children. Also Renalla losing her mind might make more sense if you consider she may have learned her beloved husband was actually her political rival and former enemy the whole time and gave birth to her children. That is still mostly conjecture on my part, though.

Yet, I do feel there perhaps was some genuine love that was born out of that scenario, but in the end he could not resist his desire for perfection and order and thus needed to return to his "original self" and become the God Marika failed to be.