i agree that having the kind of patience shimamura does is helpful, but shimamura has no empathy. she doesn't care that much. like when they had the "fight" shimamura doesn't try to understand what adachi is going through, she just says "what a pain" at adachi's outburst, hangs up the phone and forgets about it a few days later. that is not healthy at all. in real life this would be the same as trying to talk with a brick wall. the wall will listen to everything you say, but it doesn't give a shit as to what you say
I'd disagree. She has normal empathy. She acts like a normal person in a romance manga. She's just not dramatic. And of course she said "what a pain." Anyone else would have said much worse. Adachi was indeed being a pain. Just because Shimumara acts in a non-theater like way doesn't mean she's without emotion.
Also, she does understand Adachi. She understands her more than anyone else. She simply does not indulge Adachi all the time, and Adachi needs that. She has told Adachi many times that Adachi's clinging to her is unhealthy and that Adachi needs to branch out and find other friends too. Adachi takes that as a negative thing because she's always filled with anxiety, but Shimumara is showing care there. This whole trip with friends is Shimumara again trying to get Adachi to open up to other people. Shimumara is unique only because she doesn't act like an over dramatic manga character. She acts like a normal girl.
We can't lose sight of the fact that how these characters normally act is abnormal, and Shimumara is not weird for not being abnormal. She's not crushing on Adachi in a melodramatic way. She's hanging with a friend who she might like a bit but who has a lot of weird quirks that Shimumara has to manage. The LNs do a much better job of showing this as well.