He was never a bully. You could think of it as an extrovert person seeing a person who's always alone and wanted to always be there for them. It was explained in later chapters too, Ayame wanted to be close to shuu all the time to protect him from other alphas and also because he liked him. Shuu also didn't think of him as a bully but more of an annoyance, like "why is such a bright person always hanging out with me". Even in highschool, Shuu also liked Ayame a bit, he even wanted to stop wearing glasses because of Ayame but discarded that thought, and he really missed Ayame's presence when he graduated.
I'm confused. The lines were pretty clear, but it's confusing that they switched from a bully-victim relationship to loving each other so quickly. I think it's still kind of a good story though?