Oh wow, talk about a dynamic that gets reinterpreted in a million ways. In a lot of fics, their relationship starts right where the canon left off—with that messy, obsessive rivalry, Grell's desperate infatuation and Madame Red's brutal, violent rejection. Some writers lean hard into the tragedy of it, framing Grell's later devotion as a form of twisted, unresolved grief, a ghost forever chasing the one person who saw their true self and still chose to destroy it. You'll find a whole subset of fics that are just Grell tending the grave, monologuing to a headstone, which is bleak but weirdly poetic for this fandom.
Other times, especially in alternate universe or fix-it scenarios, it gets softened. Maybe they meet earlier, before Francis becomes the Ripper, and Grell's flamboyance somehow anchors or humanizes her. I've even seen a few where Grell, in their own chaotic way, becomes a kind of guardian for the memory of 'Angelina,' separate from the 'Madame Red' persona, which is an interesting angle. The power imbalance is always there, though—one is literally immortal, the other very, very dead. That inherent impossibility is what fuels most of the stories, whether they're angsty or strangely tender.
Honestly, half the fics I've read use their dynamic as a backdrop for exploring Sebastian's reactions or William's annoyance, which feels a bit like a missed opportunity. The real meat is in the silence after the scream, you know?