Gotta start off by saying that the art is excellent! The sprites have so much personality, and the backgrounds are clean and polished. No complaints there, it's all technically proficient and functional.
The presentation has some stuff I'm not as in love with. First of all, the font size is huge – I thought the game was using a mobile version of the UI due to a glitch at first. Besides hurting the readability by itself, combined with the tiny text box it means there's not a lot of space for text, which you can see with how many lines get cut off. I also don't get the point of overlaying all the prose on top of the screen instead of using the box; it's harder to read, obscures stuff you might want to look at, and forces your eyes to jump up and down.
But though the text is presented sort of jankily, I'd say the writing feels largely natural and pleasant to read. Some caveats, though: punctuation is inconsistent, with a lot of periods being omitted seemingly intentionally, and while the dialogue is pretty nice, the stream-of-consciousness narration comes off as somewhat blunt and impersonal. I feel like it could afford to be a little punchier and to do more to put you in the character's head instead of just detailing what's going on.
I guess I found a lot about the plot kind of unsatisfying for the same reason I'm not into soulmate stuff in general: I think it literalizes the mechanisms of how romance works as a genre, basically making the characters aware of the fact that they're in a story and removing a lot of the tension as a result.
The "will they or won't they?" turns into "they will". Now, it did surprise me positively that the story seemed determined to bring some nuance and complexity to the premise... but by the end, it didn't really answer the questions it raised or pull off some kind of bigger twist? The characters went on a date despite the initial hesitation essentially because fate said so, and things seemed to work out as foretold. I'm not sure if there's supposed to be more to come or what, but the ending just felt kind of abrupt and anticlimactic. The final line is cute, though.
On a String: ultimately a pretty uneven package. Some of it was amazing, some felt like the devs were still finding their footing in the medium, and some didn't quite fulfill its intriguing promises.