Pretty dumb plot point. If Heroes are capable of ending a country that has existed since ancient times, and the government are aware of their existence, it's hard to believe that there are no countermeasures in place for future Heroes going rogue.
Theory A: Some unknown girl from his class
Theory B: A prank
Theory C: The little sister; she's doing the Lelouch 4D chess video call BS where she predicted his behaviour because "she knows him too well"
Theory D: His best friend is gay and yandere for him
Mandatory main heroine GET.
It seems like the author was taking a piss on the generic goodie two-shoes overpowered MC by making him an edgy psychopath. It might be a subversion, but still a trope-y, generic story nonetheless.
I'm genuinely sick of the "I'm very weak, BUT I'm actually LE STRONG??!!" manga where everyone is completely oblivious except for the supposed "opposing faction"