Who Are Marika, Radagon and Miquella?
The Golden Order's ruling family, and the twist at the center of it.
Marika the Eternal is the god-like Empyrean who shaped the Golden Order and, by extension, most of the Lands Between's laws and religion. For most of the game, she's spoken of as an absent, almost mythic ruler - the demigods are her children, and the world you're exploring is largely the result of choices she made.
Radagon
Radagon appears, on the surface, to be a separate figure: a champion of the Golden Order who married into Marika's family and fought on its behalf. The game eventually reveals this is not the full picture - without spoiling exactly how, Radagon and Marika turn out to be far more directly connected than early appearances suggest, and untangling that relationship is one of the game's central lore reveals.
Miquella
Miquella is one of Marika's children and the twin sibling of Malenia, the Blade of Miquella. Unlike his siblings, Miquella is cursed to never physically grow past childhood, despite being (by every account) brilliant, compassionate and quietly ambitious. Much of what he's actually been working toward for centuries is kept deliberately vague across the base game, hinted at through his empty, overgrown Haligtree stronghold and his sister's devotion to him - and it becomes the central focus of the Shadow of the Erdtree expansion, which this guide won't spoil here.