Now that I have got over the box cutter development, let's talk the two new characters. I feel like I have a pretty good grasp of their personalities even with just one chapter (and the other girl doesn't actually show up). Like Kasumi, they were plagued by disappointment over their perceived lack of achievement. But they manifested their resentment in the form of rule-breaking behaviors, in this case having a secret relationship right on school ground. In Sayori's mind, she was getting back at the well-mannered girls who presumably looked down on them ("Who's laughing now?") by doing things they were not supposed to do.
While Mizuki didn't show it, she must have been frustrated over her lack of artistic talent, and the expensive equipment might be her way of compensation, but unfortunately it just earned her ridicules. Likely the same thing happened in college, and she had bottled up her bitterness for some time before Sayori announced her change of plan. Sayori noted that there were sides of Mizuki people failed to see, but she himself failed to realize the extent of her senpai girlfriend's resentment.
I notice that Sayori didn't call Mizuki "oneesama", which means they didn't stick to the roleplaying like Kasumi and Haruyo do. While the main pair idolize Class S and seek to emulate the relationships they read in the old stories, Sayori and Mizuki seemed to just use it as an excuse for breaking school rules and having fun. Of course what they have was a full blown romantic relationship striped of any pretense, as pointed out by Kasumi. The question remains, how are they still together after what Mizuki did? Sayori has every right to run for the hill, but here she seems eager to have a double date with the main characters. Hopefully we'll find out in volume 3.
Edit: I almost forgot about the letters. I made a joke last chapter about a secret society but it might be true after all?