I think Charles grins fondly when Edwin's being a bitch because he genuinely thinks it's hilarious and adorable. But I also think he admires the shit out of Edwin's unwillingness to tone himself down or file off his sharp edges. Charles needs to be liked so much, and he spends so much time setting himself aside and tying himself in knots to be what other people need, and Edwin just doesn't. He is exactly and entirely his snippy bitchy self and everyone else can take it or leave it. I think there's something exhilarating in that for Charles - both a sense that by attaching himself to Edwin he can borrow some of that fearlessness about outside judgments, and a thrill of knowing that Edwin can ignore everyone else's opinion because Charles is always, entirely, inarguably enough for him.
And there are downsides to that dynamic. Charles works really hard to manage the vibe, and the fact that Edwin doesn't really do that back (isn't equipped to do it back, because he doesn't read people and intuit what they need from him the way Charles does) sometimes leaves him feeling like he's carrying that weight alone. (who else is gonna keep spirits up? you?)
But also there's a reason Charles is drawn to stubborn difficult people with sharp edges who don't apologise for who they are, and I don't think he'd give up Edwin's bitchiness for anything.
