Really sorry for Tarumi. But she "betrayed" Shima through separation in middle school. Not imaginable for dog-girl Adachi who would turn the world upside-down just to stay next to her Shimamura.
I really wouldn't go as far as to say "betrayed", I know quite well how easy it is to drift apart at that age when you're no longer always together in class... I doubt Adachi would have fared any better, considering how long it took her to get back to talking with Shima even in the same class just because the other trio of girls started hanging out with Shima - and this already in high school. Had they been in separate classes as well, she would likely have disappeared into good old second floor of the gym, just as she almost already did, rarely to be seen even by her classmates again, unless Shima had found it in her to make a conscious effort of connecting with her once more.
Yes, I agree, it was life, fate if you will, that separated Shimamura and Tarumi. So to clarify my point, I didn't really intend to put the actual blame solely on Tarumi, which is why I put "betrayed" in quotes. If I got it right, however, this separation is what triggered the biggest change in Shimamura. She lost faith in strong relationships, lost faith that it's even possible to have a relationship that would withhold life obstacles. And she stopped investing in relationships. So the way I see it, sort of from Shima's perspective, she's experienced some form of "betrayal" from Tarumi's younger self. I don't have a better word for it.
Would have Adachi performed better, had she been in Taru's shoes back in the day - who knows, we'll never know. What we do know, is that Adachi of now did manage to come back to Shimamura each and every time, no matter what, like Gon. And this persistence and devotion of hers is what, step by step, made Shimamura fall in love with her. Shimamura has never had a chance to experience this sort of "betrayal" from Adachi. Yes, different situations, age, time, everything. Not fair to compare with young Taru. But that's how life is. For Adachi, Shimamura is everything, prio number 1, no, prio 0 even. And she proves it every day. And she wouldn't let Shimamura send a message "Just got here" on a date - Adachi would've been there an hour earlier.
The really interesting part is that I think Taru did have quite a chance back in "Spring and Moon", but it was Yashiro who timely interfered. Had Yashiro not popped up out of thin air, I wonder how would things played out. We will never know. Fate.
