These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Iris Zero
Fridge Brilliance: When Houjou shouts at Sasamori that she as a School Idol will never understand how hard it is to confess, he doesn't refer to his confession to Kuga, but to his unrequited love for Sasamori herself. He is furious because he will never be brave enough to tell her and feels Sasamori mocked him about it. It also gives a new view on his anger toward Toru. Toru's the guy who ruined his relationship with Kuga and, despite being on the bottom of the Popularity Food Chain, manages to get Sasamori's attension, when Houjou had to work hard to get anywhere and still failed.
Ho Yay: In the omake of volume four, Tokita's idea of extending the series involves pairing him with Hijiri (sparkles included).
Jerkass Woobie: Despite some disgusting behavior Kuga is living under an impending death sentence and has alienated all of the people who can help her, well, most of them...
Houjou, once backstory and characterization set in post-Iris Hunter.
Nightmare Fuel - Hiziri's Iris: he can see how close a living thing is to its time of death in the form of black butterflies. This caused Hiziri to become apathetic about life, including his own. A certain situation that makes this truly horrifying is when Hiziri sees Tachibana practically engulfed by a swarm of black butterflies, here.
Tear Jerker - Hijiri's story of how he and Toru met leaves Sasamori almost in tears at one point, but what really drives it home for her (and potentially the reader) is his question and Sasamori's answer afterwards: "Was I qualified to save Tachibana back then?" Sasamori turns away in tears, overwhelmed by the fact that she can't tell him "yes," and the look of sad, futile resignation on his face at her non-answer.
Kuga's backstory was so soul-crushing that her childhood friend Tokita resolved to make her happy, no matter what it takes. And I bet the reader would, too — no child should ever have to go through something like that.
The Woobie - Toru, especially in flashbacks. He's got some good friends now, but the rest of the world largely thinks of him as something to ignore or defecate upon. And guess what, having Sasamori has a friend made things even harder for him.