This guy has had a crush on the MC since childhood. Everyone knows it. The MC knows it.
But, then, the MC can choose to be horrible to Ostric, siding with his toxic parents over him.
If the MC does it, Ostric says "I'm done." Which, yay, good for him!
But....if the MC continues choosing and pursuing Ostric (or if they had a strong history of doing so), he comes back. He gives the MC another chance even though he knows it is a bad idea.
He tells the MC they're not someone he can get over.
So I have two options here.
The MC ends up apologizing and they move to a romance route.
The MC realizes just how much power they have over Ostric and I lean into that. And they stumble their way through figuring out the boundaries. The MC able to be mean, trusting Ostric knows they still choose him and want him, that he won't walk away. Ostric taking every hurt, getting power over the barbs he's received his whole life because when the MC says them, he knows it means he's being trusted and chosen. And it hurts, but the MC soothes it with presence and kisses and affection, too. It's like lancing the pain, kinda.
1 is the safer route by far.
2.... some readers might like that. Instead of an apology scene and moving to equal ground, it becomes figuring out their boundaries together. For example, the MC might promise to never team up with Ostric's parents again, even if they make similar comments. Or they limit the insults and such to a specific time/place. And then finding that tension between 'they don't mean this' and 'they mean this and they love me anyway/because of it.' And the MC enjoying Ostric's reactions as they verbally flay him open.
Most readers probably wouldn't enjoy that, would they?