1. The fap incidents are serious enough to warrant death threats and even the MC was forced to apologize to another student and their parent, but literally years of bullying Kitahara was not serious enough for any teacher to care.
2. Nagaoka and Takigawa are supposedly "friends" with EVERYONE, but neither of them ever defend Kitahara or even address her problems.
3. Oh, remember how many of us bullied you, Kitahara, for years to the point where you quit school and became a shut-in? No? Neither do we. Boy are we glad you ended up coming to the reunion at the incessant behest of Kurosawa though! Everything's resolved!
Basically Kitahara felt largely used as a tool to advance the rest of the characters, and may as well never actually existed as a character. I just feel unsatisfied with her (lack of) development, and while I think the MC's story concluded nicely, it still felt tainted overall.