I'm afraid I don't know what you mean by red flags? The relationship between Isabelle and Cy is a remarkably healthy one! If you're talking about my recent post about how Isabelle's type is essentially just really toxic individuals, Cy is honestly a deviation from her type, at least in how he treats her. They bond as friends before they're D/s. The core of that platonic bond is (in my opinion, Null's might be different) the way they're both sort of alien to most of the people around them, vulnerable in some ways but closed-off in others. Isabelle doesn't find out about Cy's past crimes until well into their relationship.