This volume gets to the core of what I love about this series, which is how believable and lived-in the relationships between the characters feel.
This volume starts a two-part mini arc about Haruna, Renako's little sister, as she goes through some shit and how Renako tries to help her through it but has a hard time getting through her walls to do so. It also delves deeper into what exactly Renako's high school life was like, including quite a few CWs for bullying, so be aware of that going into this one. It can get dark, especially for a series as fluffy and comfy as this one (though if you've read volume four you know that this series doesn't shy away from frank depictions of depression and more mature subjects)
This volume ends on a bit of a cliffhanger (one that doesn't depict one of the Quintet in a particularly favorable light), so have the next one ready when you finish it!