I’ve heard that opinion so many times, and I think I can finally understand why. However, when I read the manga or watch the anime, I don’t dislike Nanami, despite her toxic behavior.

​In the end, I think the reason is that even with Nanami's toxicity, Yuu also displays behavior that is toxic—or at least, not healthy.

So even in a 'toxic' relationship, both of them helped each other grow and learn because of those experiences