or she just too simp, many such case.
Thing is, I don't think any of the characters in this story are meant to be all that bad of people. There's just circumstances and what's presented. A lot of what we see in the manga is flavored by Kyo's perspective, so of course we tend to get a negative impression of characters like Nanjou, and assume something demeaning about those that like them.
At the same time, it was made clear early on that if Anna was up-front with Nanjou about her lack of interest in him, he'd leave her alone. The problem was, she has long been awkward around boys, with the exception of Kyo due to various factors, including how he was far more appoachable than any other guy that was interested in her, but she also had to be careful how fast she got close to him, which has been a major focus of the story.
In any case, her awkwardness caused her to never clearly reject Nanjou, so he never really gave up... until the incident at the end of
Ch75, where Anna drags off Kyo and gives a very clear 'sayonara' to Nanjou (that 'sayonara' note in the Boredom Society translation felt very poignant). At that point, it started to sink in how she wasn't interested in him at all, and clearly is attached to Kyo, and their feelings were mutual. He was accepting that they were becoming an item already and that he was barely a blip for Anna.
The way I interpret his last line during that chapter is him realizing he has a month left to settle all of his matters in middle school before he graduates and leaves everyone there behind for good. This also explains the idea of him 'getting serious' (according to the anon translation of the parting note for the chapter), as he's probably done with 'superfluous' relationships, maybe spurred on by seeing Kyo and Anna together.
This means, among other things, breaking things off with his current 'girlfriend' (
Ch82, Pg8) who he is clearly not serious about, who assumes that Anna is the one he is serious about, which is probably partially true, but it's likely more complicated than that. He's probably harbored feelings for Anna for quite awhile at that point, so in that sense it is true, but I do wonder if he's already starting to realize how important Mamiya is to him, as well.
This also explains why
Ch85 is merely 'ceremony'. He even uses 'ceremony' to describe the confession himself when it's over and he's leaving, though I can understand if people misunderstand that he was only referring to the graduation ceremony, but it had a double-meaning.
In conclusion (
tl;dr, if you will), this summer festival arc with him was merely him dealing with whatever lingering feelings he had for Anna after obsessing over her for so long, and getting closure, so he could finally get serious about Mamiya, who I think was who he really meant in Ch82, but the girl he was breaking up with there just assumed it was Yamada, and he rolled with it, especially since he was probably already planning to make a 'show' of confessing to her after graduation to try and push Kyo and Anna, because he actually does low-key cheer for them, I think.
