He's drawn to him, Kamui to Fuuma. It's inevitable, the pull between light and dark, "good" and "evil." Neither one of them is really certain who is "dark.".
If they had to guess, they'd both guess Kamui. Fuuma's always been the victim, ever since the first. It was his mother who died for Kamui's mother, who dies for Kamui. It was his sister who was loved by Kamui, only to become a pawn between them.
It was his soul tied to Kamui's, and it wasn't even his choice. It's never been his choice.
Kamui got the choices, Kamui got the love. And in the end, it was Kamui's choice that made Fuuma who he became.
So possibly, it's Kamui's fault that Fuuma treats him like he does. The mind games, the torture, the sexual taunting. All of it going until they aren't even sure who's top and who's bottom. And Kamui, when he remembers, is appalled at himself. And yet….
And yet, somehow he's stopped minding. As the dark Kamui, Fuuma has power, class.
As the Dark Kamui, Fuuma has no more inhibitions. As the Dark Kamui, finally, Fuuma has Kamui. Kamui has Fuuma.
After all, they're not that different.