In hindsight, Masamune really should have realized it sooner.
Right from the start it should have been obvious that those bandages Kyouya wore, were not to treat injured ribs. If he'd been paying any closer attention he'd have realized sooner that they were crude and not professional. They weren't even wrapped the way they should be to actually help. Hell, they barely even covered all his ribs.
But, in his defense, Masamune didn't intent on dedicating a lot of time staring at Kyouya's chest. Sure, the sight wouldn't have been bad, but it would have been weird regardless. Even now that they're friends (acquaintances? Masamune wasn't sure how close they really were, though Kyouya at least liked him enough to pay him a visit when he was in America), he wouldn't just stare. It was rude and he wouldn't even know how to explain himself when Kyouya inevitably noticed.
Now though? Standing in the changing room of the Dungeon Gym, Masamune noticed clear as day that those bandages Kyouya wore were not because he hurt himself. He knew Kyouya hadn't even intended for him to notice, because he hadn't even intended to change, but his shirt got dirty, and there was really no way around it, especially when Coach Steel had so graceously offered the other boy a shirt.
"You're binding." "What?!" "Those bandages. You're not hurt, you're binding."
Saying it out loud now, somehow, it seemed like the most obvious thing in the world. With the way Kyouya's voice occasionally cracked. The secrecy with his clothes, the coats that hid his hips. His features. Suddenly everything made sense.
Kyouya, on the other hand, was not at all thrilled with Masamune's sudden realization, and he glanced around quickly, seemingly making sure they were completely alone.
"How did you find that out?!" Masamune scoffs at the question. "The way you bandage your chest. It's crude. It's not even reaching your ribs!" He sighs at that. "Do you know how unhealthy that is? Why don't you use a binder?!"
The realization that Kyouya was trans really didn't shock him as much as the fact that Kyouya was completely reckless with his binding.
Immediately Kyouya assumed a defensive position, pulling his shirt over his head and crossing his arms. He had his back turned to Masamune now, and Masamune felt frustrated by the simple notion.
"It's not that easy when you don't have someone that can get it for you," Kyouya huffed eventually and Masamune quickly got the hint that Kyouya's family probably wasn't the most supportive. It's not as if Kyouya could just randomly order one, either. A frown spread over his face and he thought for a moment.
"Well," he began, a little sheepishly. "There's a store nearby. We could go and get you one."