Pretty interesting read.
This went way darker than I expected given the song, and way different.
Unlike the song I feel like this paints Miku to be much more at fault (and crazy).
The song I feel implied much more cheating/deception/betrayal, yet here it's just the fact that a promise was broken. Miku and Kaito never even dated. And Luka actually told Miku the truth when confronted. While it's still understandable that Miku's mental would be destroyed, it seems like she really overreacted.
But fine, promises are really important, and judging things like mental states is subjective. And the deep loneliness that (supposedly) woul await Miku was pretty drastic. So it makes sense she would go a bit crazy. Especially after Kaito "held" her. (Side tangent: why can't they say "slept with" or just sex if this manga has self harm and suicide?).
I was amazed at how broken Miku became with how she acted afterwards. Miku was really obviously in need of dire support. Yet Luka and Kaito don't do a single fucking thing. Only at the night of the stabbing do they decide on staying by her side... but that's all. They'll only be there until she gets magically "fixed" (quote: "lets wait patiently until time heals her"). I think considering Miku is literally hallucinating/signalling she wants Luka out of the picture, they should realistically be MUCH more hands on than just that. And a bit earlier too.
Maybe you could say this shows how Luka and Kaito care for eachother first and foremost, and that Miku is an afterthought. But that's really cruel (likely not the case...)
Also I feel like the ending was a bit of a cop out. The manga was surprisingly okay with being incredibly angsty and tragic so far. I didnt expect it to get so dark, so having it not end like a tragedy is strange, since unlike other dark mangas where people (stupidly) die in the end, here it's sort of expected that miku and Kaito would die. So yeah... they live. And what then? Done? It's over? I for one think we should torment Luka for eternity for the great sin of hiding a relationship Kaito initiated with her behind Miku's back for a while. While Miku wasn't even like ever dating Kaito or anything.
Luka really was a victim. She couldn't really do anything. Except confess to miku earlier. But that's also hard for Luka, and... Kaito isnt THAT much at fault since he didn't know of the promise (that Luka and Miku would not have a relationship with Kaito). Though he did create a deep fracture. I feel like Miku just really was done dirty here, she just became murderous over so little. It should have been balanced better.