Read this all in one sitting and dropped it. I'd give it a 4/10 right now.
My main issue with the story is that it feels very meandering. I was still on board when Hikari saw them kissing, that sort of drama is fine, but then we cut away from the drama to an extended flashback. I don't think this was necessary or well placed. I don't think that Yuu or Aya actually need to be justified in their actions, so I don't need to see how sweet they were being, and the thing I was presently interested in was interrupted. But then, even after we return to the present, or rather, to Hikari's story, her actual weight in the narrative doesn't change much.
Even if she refuses Yuu's confession because of the kiss, I can't really care very much anymore, even though I should have been about it. There was simply too much of a delay between the two, all spent on a flashback of a character I had little investment in. The tension of seeing the kiss was basically entirely undone by a flashback that was more than anything just mildly discomforting. The only part of the flashback I had any real interest in was the moment in the manga cafe in which Aya takes the place of Hikari, but that angle, in which Aya was filling the role of Hikari, isn't hammered into as much as I might have enjoyed (though the latest chapter as of writing this, 51, seems to have returned to this idea).
My lack of investment in Aya's character and past also stems from the fact that Hikari is cut out of it, we are thrown into the flashback before Hikari even processes what happened. I really think that the story would have flowed better if we were to see the flashback when Hikari learns of it properly, if only because then we could actually sit with her feelings. But even then, after the story returns to her, we don't get much. It feels like within the larger narrative, the author no longer knows how to depict the relationship between Hikari and Yuu. There isn't much actual drama between them, nor is there much dwelling on their feelings once the truth has come out.
The drama doesn't concern either of them that much, and especially not their relationship, but rather Aya and her character. And in terms of "meandering," this drama fulfills the same role as an interrupted confession in a romcom. The main couple isn't really being built up, but rather something else is happening to postpone that building. As long as Aya's drama is the focal point of the story, the relationship between the two is at a standstill. Nothing can happen until things are brought to an end, but it simply takes too long. There were 20 chapters dedicated to Hikari and her relationship with Yuu, and 30 longer chapter dedicated to Aya drama and the sinking into her darkness (though chapter 51 also seems to be signaling a conclusion is close).
In short, because of the way the story has been structured, the ending is already a foregone conclusion: I cannot bring myself to be interested.
The most "traditional" way to end it would be for Hikari and Yuu to end up together (and even textually this is where I would assume it to be going; between the idea of "replacement" and that even while dating Aya, Yuu seemed to be longing for Hikari it seems to be the path (and of course that he confessed to her even after seeing Aya again)), but this is problematic for several reasons. If they just get together then it's a repudiation of the drama story that steps on the dark emotions that have been festering in the characters (though in their recent interactions it's already done that in many ways, which has already left me unsatisfied). But if there must be a dramatic conclusion for them to start dating, what is it supposed to be? There isn't enough of a dramatic tension between them outside of Aya for there to be something to resolve, and introducing it after resolving the dramatic tension of Aya would be too little too late. More than that, my actual investment in their relationship waned dramatically because of the dominance of Aya in the narrative. Though that leads to the next possible ending: an "Ayami Ending." This ending is also quite sensible in a pure narrative sense at this point, and would complete the tragedy of the title, but it's also problematic. For one thing, it brings into question why we were ever supposed to be invested in Hikari and her love in the first place. It begs the question of why the story was framed as it was for 20 chapters if Hikari really was a losing heroine who was already too late (especially because textual support for this being true is, imo, quite weak, unless Yuu chooses Aya right now). Or rather, why have Yuu continually choose Hikari over Aya if it were to end in this way. Beyond that, Yuu and Aya are in many ways framed as a dark mirror of Hikari and Yuu, and their relationship is built upon that twisted foundation. In terms of the NTR conversation, the flashback really is BSS, as it really should be the case that Hikari is where Aya was, at least in terms of how it is framed. Just on this basis the ending could only be twisted and, as with the flashback, discomforting, which ultimately returns us to the first point on this ending. A harem ending is out of the question, I feel, because it would be a return to a status quo that never existed in the first place, or rather, Yuu would continue to be gutless, weak, and indecisive (despite having made a choice already) despite the drama of his character being that he wants to change from being that (barring some miracle of writing). A "nobody wins" ending is perhaps the least problematic, but in terms of an emotional resonance, hinges on something that no longer exists. This ending works best if it can empty the reader out--and from a thematic sense I hold the belief that a strong emotional reaction in the audience would heighten the ideas of being "too late"--but, in the end, I don't hold enough goodwill towards either couple to be emptied out anymore. Though of course, I have little desire to see how it ends.
The story has good ideas in essence, but they are severely hampered by what I view as an ardent belief in the dramatic turn being a twist instead of a development. The story feels disjointed and meandering, the dramatic elements becoming a way to avoid and push aside the Hikari/Yuu couple that was introduced as the focus of the narrative and its emotional core rather than a method to examine it and advance their narrative, which in turn fails the Aya dramatics at the same time.