Volume 5 primarily works through the friendship problems between Bon and Sen from the prior volume. Though it's not my favorite arc, I do think it's a story line, because it deals with Bon's jealousy over his first real friend making new friends. It's not a flattering emotion, and it's petty as hell, but it's a real thing, particularly if you're someone to whom friendship does not come easily. Ask me how I know.
The volume also gears up towards a major conflict between Sen and Jun-nii over their shared love interest. There's still not enough detail in this plot line for me to know how I feel about it entirely. Jun-nii seems like a creep thus far, but they all seemed to respect him before this. I'm also not sure how old is he is exactly, which impacts the level of ick factor in his relationship with a high school student.
Definitely an interesting volume, if a bit weakened by the lack of an individual arc. These volumes do not really have story arcs but just contain one chunk of the larger story. They'd be stronger if they had their own mini-arcs to go along with the larger tale.