So I'm currently still working it over in my brain, but I think they WERE friends as children, but after Events Happen, Nai just becomes more and more of a manipulator and has this hatred for certain humans (there's two sects of them in this AU).

When he and Legato are teens, he knows Legato likes him romantically, so he uses this to his advantage to manipulate Legato to do what he wants. Mind links don't allow actual physical control, but imagine having someone like Nai in your head all the time. 😭

I don't want Nai to be a "villain" exactly though, like he still thinks he's the hero of the story despite how he treats others.