Which is bound to fizzle out??? Apparently every young couple who faces any awkwardness is doomed now.
I like the “alien child” (whose name is Yashiro, you’d know if you payed attention to the story). You’re coming at this as if the manga has to prove something to you instead of reading it for fun. Room temperature take

I mean yeah, most young couples are doomed. Most couples period are doomed. That's not necessarily a bad thing, and it can be interesting to read about couples that might not work out.

Given this story is highly introspective and obviously trying to elevate itself above the typical surface level plots of many other mainstream romance manga, I think it's pretty obvious the author is trying to prove something. Which, good! Easy, fluffy, rote stories can be comfy, but I'd much rather read something by someone who feels they have something to prove. However, this naturally invites readers to more deeply analyze the story. Which is also good! Engage in dialogue with the stories you read!

I think the characters are very well written, but the overarching narrative and a few of the specific decisions made, at least in this adaptation (again, I haven't read the LN and fully believe it might be done better there), feel incongruous. If the narrative hadn't directly spelled out that these characters were supposed to be soulmates, I would find a relationship between these two kids trying to navigate growing up while dealing with these enormous hidden feelings more compelling. I don't (yet) buy that their personalities are a good match, that they can truly mutually support one another emotionally, that they're willing to grow for themselves as well as each other rather than just enable this codependence they seem to be developing. Like I said, maybe that growth will come. But they're kids! I wouldn't expect them to be capable of that level of relationship at that age anyway. Personally, I assume that lots of the relationships I read about in manga - especially school age manga - will end one day, but that the characters will have grown as people as a result of those relationships.

As for Yashiro, she has yet to contribute to the narrative as presented by the manga. I'm not saying you're wrong to like her, but her presence or absence in the manga would have no meaningful impact on the story or characters thus far. I hope this changes, but to me, she comes across as an unjustified gimmick.

Which is bound to fizzle out??? Apparently every young couple who faces any awkwardness is doomed now.
I like the “alien child” (whose name is Yashiro, you’d know if you payed attention to the story). You’re coming at this as if the manga has to prove something to you instead of reading it for fun. Room temperature take

I mean yeah, most young couples are doomed. Most couples period are doomed. That's not necessarily a bad thing, and it can be interesting to read about couples that might not work out.

Given this story is highly introspective and obviously trying to elevate itself above the typical surface level plots of many other mainstream romance manga, I think it's pretty obvious the author is trying to prove something. Which, good! Easy, fluffy, rote stories can be comfy, but I'd much rather read something by someone who feels they have something to prove. However, this naturally invites readers to more deeply analyze the story. Which is also good! Engage in dialogue with the stories you read!

I think the characters are very well written, but the overarching narrative and a few of the specific decisions made, at least in this adaptation (again, I haven't read the LN and fully believe it might be done better there), feel incongruous. If the narrative hadn't directly spelled out that these characters were supposed to be soulmates, I would find a relationship between these two kids trying to navigate growing up while dealing with these enormous hidden feelings more compelling. I don't (yet) buy that their personalities are a good match, that they can truly mutually support one another emotionally, that they're willing to grow for themselves as well as each other rather than just enable this codependence they seem to be developing. Like I said, maybe that growth will come. But they're kids! I wouldn't expect them to be capable of that level of relationship at that age anyway. Personally, I assume that lots of the relationships I read about in manga - especially school age manga - will end one day, but that the characters will have grown as people as a result of those relationships.

As for Yashiro, she has yet to contribute to the narrative as presented by the manga. I'm not saying you're wrong to like her, but her presence or absence in the manga would have no meaningful impact on the story or characters thus far. I hope this changes, but to me, she comes across as an unjustified gimmick.

I think a lot of the issues you have w the manga get ironed out in the LNs. Shimamura's internal monologue gives a lot of insight into her as a character and how she views this relationship, and while they do their best to put the super important parts in the manga, there's a ton of stuff that's omitted. Similarly as pointed out, Yashiro's character gets a lot more attention in the manga and the world just in general feels more alive if that makes sense (hino and nagafugi get a lot more focus, even receiving entire chapters dedicated to their goings-on). Also like, this series is all about how adachi and shimamura's relationship and how it grows into something beautiful, as well as watching how the two progress as people, it definitely takes time but watching them get to a healthier place is so definitely worth the payoff. If you are interested in reading the LN's there a really good translation by Sneikkimies online that has all of them translated (except SS2 the most recent one it's only half finished)

Omg ive been waiting for the yashiro bag part FOR SO LONG its finally in the manga...

Meta-wise, Yashiro also provides a connection between this series and the author's previous work, Denpa Onna to Seishun Otoko.