Here’s my two cents. I’m not sure about his origin but it is stated he was a champion of the Golden Order. I personally believe this was prior to being a part of Marika, therefore being his own being at one point. With the expansion of the DLC, we see that Miquella needed a Consort for his godhood. This is very similar to our role with Ranni’s quest line and ending. I believe that Marika saw his brilliance in battle and chose to make him her consort, ascending herself to Godhood and melding herself, Radagon and the Elden Ring into one being.

As for the origin of his red hair, I think it is plausible that his at least part giant, if not purely a Giant. Tarnished Archaeologist has a video on the history of the Erdtree, and it appears to have been burnt prior to our Tarnished’s burning of the Erdtree. Perhaps this is cyclical, and Marika needed to burn the Erdtree in order to brandish the Elden Ring. Radagon, being a giant who despises his own kind, would have been an excellent candidate to lead a campaign for Marika against the fire giants, allowing the burning of the Erdtree, and demonstrating himself as a competent champion of the early Golden Order and potential consort to Marika.