They aren't siblings, don't worry.
Ameri's twin shows up later, and he literally looks like Ameri with black hair. He's gay and kinda cunning too, thus the title of this comment lol.
And Koyama is actually Ameri's ex bf. Legit ex bf. Like legit, legit. Legit gay.
Yuuma ofc lol. After Koyama moved to other school, he met real Ameri and our MC. 3 of them became close, Koyama told them about boy called Yuuma, our MC immediately knew that's his Yuuma, so he borrowed Ameri's identity and enrolled to the same HS as Yuuma to save him. Real Ameri is okay with it cause he has some family issues, also a shut-ins.
Fake Ameri a.k.a our MC is Yuuma's childhood friend named Mao but because there was another Mao, the girl one. Yuuma completely forgot about him, his memory about childhood friend named "Mao" overlaps. So Ameri keeps pretending.
We're finally back to the main plot. I just hope Yagi get plot armor too. Kinda scared something might happen to him especially after it was implied that his father was like him.