This manga is so odd. The strange alien child adds nothing and frankly just detracts from the otherwise grounded story, meanwhile the MCs' relationship has very little chemistry and often feels actively unhealthy. It has its cute moments I guess, but to me it feels more like a story about a fairly realistic childhood relationship between two kids who are just trying to figure themselves out and which is bound to fizzle out in relatively short order, rather than about destined soulmates as the narrative claims. I haven't read the LN and maybe some character development will eventually change that impression, but it's going to take a lot to turn it around.

The main problem about yashiro the alien child is that you mostly see her in the LN intermissions and side stories where she just be doing whatever and she hangs around shimamura's sister and family for fun and sometimes says profound stuff. The manga adapts none of the intermissions or sometimes they end up as 1-2 page volume extras so yashiro feels really out of place in the manga version.

She really gets her time to shine in LN vol 99.9 and she genuinely becomes one of the best characters in the whole series in that volume. We still don't know if the manga will adapt 99.9 though.