Many people relate to Adachi. Am I the only person who can relate to Shimamura?
No, me too.
I have to admit I'm also fairly passive on my personal relationships, but not really apathetic to Shimamura's level.
I've read too many people blaming it all on Shimamura and I just can't agree with that. She clearly made a lot of mistakes. Because she can't see the red flags, she goes along with everything that Adachi wants. And that only feeds on her unhealthy attachment.
In her mind she's just trying to please her friend. You can't really blame a teenage girl, especially one like Shimamura, for not expecting the disastrous consequences of that. But you cannot make her responsible for her friend's mental health.