I'm honestly more interested in the dynamic when it's framed from Brian's perspective. A lot of fanfics make Dexter the obvious protagonist, but the ones that flip it—where we get to live in Brian's head, seeing his genuine affection warped through his own pathology—those are the ones that get under my skin. They're not just 'evil loves evil.' It's about a person who feels a real, possessive, obsessive brotherly love, but the only language he has to express it is murder. He wants to share his world with Dex, but his world is a charnel house. That dissonance is where the real tragedy is, I think. Some writers nail the pathetic, almost childlike need beneath all that ice-cold competence.
I read one once where Brian kept trophies from Dexter's childhood, like a lost mitten or a broken toy, and treated them with this weird reverence. It wasn't romantic, but it was deeply, disturbingly intimate. The relationship is less about romance for me and more about a twisted mirror of family bonding. They're two broken pieces that can't actually fit together without drawing blood.