I think that Masato was the most wingman-like buddy among all Key characters who were in a similar position to the respective protagonists. He’s always shown that he cares for Riki, lent him an ear and gave him an advice when he was stuck on a route in most cases (except, for example, Haruka’s route, where Kengo did that part instead) and he followed up on his words with actions if need be. If Riki was sheltering a girl in their room for some reason, Masato just went somewhere else without being asked. When Riki came back home soaked, with a mind-broken Komari and a dead kitten, Masato just took the kitten and went out (into the rain) to bury it like it was the most natural thing to do.
Masato had chosen his role, in the imaginary world: He decided to remain as he was prior to the accident and ensure that Riki and Rin would have a lot of fun when he’s around. He stuck to it and he executed it perfectly and in every situation. He was silly and funny when everything was ok. He gave Riki little pushes when there was something to take care of (Like when he revealed to the guys that Riki has been confessed to by Suginami) and he’d support him when there was trouble.
I’ll be honest. It was a very shocking moment when I realized that Masato had known everything from the start. I was like “Oh my god! He knew ALL of it and he STILL chose to act the fool. That experience must’ve totally changed him, but he behaved the way he always used to, just so Riki and Rin could be at peace and live a happy, normal everyday life for a while longer.” And then it was like Dominoes falling down one after another. Of course Rin couldn’t knock out this mountain of muscles as easily as he always made it look like. He just always acted that way, never striking back, cause it was more fun. Of course he’d grab the most ridiculous and harmless weapon in the battle rankings when facing a weaker opponent.
He decided to remain the lovable muscle idiot and his act was perfect. I can’t even imagine how hard that must’ve been. Even the stoic Kengo, who spent his life honing his body and mind, had weakened considerably - he was desperate - after the incident and assumed a rather naive position in the matter. Even Kyousuke became more cruel and calculating in order to further his own goal.
But this Kinniku baka always remained a supporter of Riki’s. He never interfered and he never tried to push his ideas onto Riki, letting him do his own thing. He always remained neutral and acted like nothing had happened all the time. And for that act of his, for that dedication, Masato has my utmost respect.
If I were the protagonist of any kind of fictional work, I’d want Masato by my side. I’d want this kinniku baka to back me up at all times like he did for Riki. I’d be hard-pressed to imagine a more reliable supporter.