I think you missed
@Rafalga's point; he's complaining about Kanesaki being a loose end which then goes against the principle of
Chekhov's Gun.
If the author wanted Kanesaki to be irrelevant to the story then they did not need to give him a face and a name. He could've easily been replaced by a generic game of telephone gone wrong and we'd still arrive at the same result.
But the betrayal makes it so much more meaningful. He lost his entire immediate family, his "best friend" he trusted with this traumatic secret viciously stabbed him in the back, and then most of the student body took the rumor at face value and either saw a murderer or an outcast to be pitied. If it were "just" telephone gone wrong then it would hurt, sure. And he'd probably still internalize some of that derision and blame, yeah. But that cruel betrayal that took his trauma that's maybe weeks old at most, and then
weaponized it against him in a way that didn't just twist the knife but outright gouged the wound?
That is how you get someone whose faith in humanity is so numb that he neither bothers with the opinions of others nor allows them to get close to him. And in that case, Kanesaki served his purpose. People want to see him get punished- be it systemically, socially, karmically, or physically- but y'know what? "Living well is the best revenge"; MC can start a new chapter of his life via a happy relationship with the FMC, someone whose emotional support and validation will do wonders for his mental and emotional health. And Kanesaki can become just a bad memory left in the dust.
