For me it makes this story even sadder than it is that Tsugumi died with so much regret. Without having been able to confess her feelings towards Midori or a separation forced by events.
We don’t know that Tsugumi died with regret about Midori, or about anything else.
We do know that Tsugumi felt more for Midori than Midori realized at the time—Midori knew all along that she felt that Tsugumi was special to her, but she assumed that she wasn’t so special to Tsugumi, since Tsugumi had so many friends, and especially since her friend disappeared without saying anything.
Now, years later, the fact that Tsugumi kept her phone number and the fact that Haru remembers Tsugumi talking about Midori fondly tells Midori that Tsugumi did think of her as a special friend, but that’s it.
It’s a bit interesting that the former classmate-now-teacher had no idea that Midori and Tsugumi were close, but that’s not so surprising given that Midori seems to have devoted her life to becoming invisible socially.