"Hikaru", the neurodivergent monster
Several posts explain how "Hikaru" can be read as an autistic character, or as someone who displays symptoms of ADHD or BPD. Here, I would like to focus on the broader idea that "Hikaru" is an excellent representation of neurodivergence, whatever form it may take, because the "symptoms" of neurodivergence are the product of a mismatch, a misalignment between the individual and the world they have to live in.
In the same way that some people reject a gay reading of the relationship between Yoshiki and "Hikaru" because "Hikaru" is not human (and therefore not truly a boy), some argue that we cannot diagnose "Hikaru" with anything because he is not human (and therefore not subject to the same emotional and cognitive processes). But the fact that "Hikaru" is not human should not prevent the viewer/reader from seeing themselves in him, from identifying with him and projecting themselves onto him. After all, one of the manga’s themes is precisely that "Hikaru's" "monstrosity" doesn't place him as far from humanity (from us) as it seems.
"Hikaru’s" "monstrosity" works as a perfect metaphor for neurodivergence for three reasons. The first is that his "monstrosity" is not a choice: it is something that exists independently of his will and something he has to live with while trying to belong to a world that was not made for him. The second reason is that his "monstrosity" is inseparable from who he is: his experience of the world happens through and with his "monstrosity", in the same way that neurodivergence is an essential part of who a person is. The third reason is that "Hikaru’s" "monstrosity" mirrors the way neurodivergent people are often perceived: strange, out of place, outside the norm.
When Yoshiki tells "Hikaru" he doesn’t want him to force himself to become human, it can be understood as Yoshiki not wanting "Hikaru" to betray who he is just to fit into a box or blend in. Even if staying true to himself means he won’t fit into society any better, Yoshiki likes “Hikaru” that way. This is why "Hikaru" being accepted by Yoshiki takes on such a special meaning: he has found someone who doesn’t force him to fit into mental patterns that are foreign to him, someone who allows him to be himself without masking (while with the other classmates, he tries to conform to the image they have of the old Hikaru), someone who appreciates him not despite his difference, but with it.
Being accepted helps us accept who we are, stop blaming ourselves, and stop seeing ourselves as a mistake. If someone loves us as we are, then maybe we do have a place here after all. This is why "Hikaru" sees Yoshiki as his home, the only place where he feels he has a right to exist as he is. Because in a sense, to be part of this world is to be part of a community, and living alone feels pointless, almost defeating the very purpose of life.
It’s also why "Hikaru" is particularly attached to Yoshiki, almost obsessed: when you’ve never been desired and have even been rejected for who you are your entire life, the moment you finally experience someone’s acceptance, it becomes hard not to cling to it, even to the point of addiction.