Isabelle and traumaswap!Cy would have a different bond than Isabelle and canon!Cy mostly because of the ways his trauma gives them more in common and the way it affects his sexuality. traumaswap!Cy is less sadistic and dominant than canon!Cy, so when he and Isabelle do kink together it tends to be less of an actual D/s dynamic and more two friends finding catharsis and release. Neither of them are strictly Dominant or submissive towards each other, but they'd find a kind of natural ebb and flow of who inflicts pain and who endures it depending on what they're feeling. Sometimes Isabelle slices into him, sometimes Cy puts out cigarettes on her, they've got a vibe going.

Ultimately, though, they'd just be friends with strange benefits. I think Isabelle and Cy both keep sex and love very much detached from each other, and while Isabelle might crush on Cy a little here and there, she wouldn't want to turn the relationship into a romantic one. She'd see him as a rare oasis of understanding and she wouldn't want to do anything to fuck that up or complicate it. Why mess with a good thing, especially if the good thing involves getting to work out both sides of her sadomasochism?