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: Yandere Kanojo
Badass Decay: Happens with Nanami, who turns from The Rival to whipping girl in short order. The author mentions this in an aside as being what caused him to introduce Sumika.
Many fans consider her confession to Shiratori on national TVthe crowning moment of awesome for Hijiri.
Chapter 47.1 has one for Nanami. Nanami hears Reina and Sumika arguing, and shows up in time to see Reina get pissed off enough to hit Sumika — in past meetings, Reina would restrain herself from beating up Sumika whenever the latter insulted her because Sumika can't fight. The sight of Sumika's bruised face causes Nanami to go ballistic, and she punches Reina in the face so hard that it knocks the latter off of her feet, something that Nanami was never able to do before. Even though it turns out Reina and Sumika staged the argument at Manabu's request in order to snap Nanami out of a Heroic BSOD, it doesn't take away from Nanami's accomplishment, because she wasn't in on the plan.
Strangled by the Red String: Discussed. One of Shiratori's concerns about his relationship with Hijiri is that he and Hijiri like each other because there is no other person available, endangering their relationship in the future.
The Woobie: Reina's dad. You just have to feel so sorry for the poor guy.
The story of how he and Ran met cements it. He tells her that she was the first person to want to be his friend. They met in high school.
Manabu's (usually suppressed) feelings of worthlessness due to Parental Abandonment. It's made even more poignant when it does show because of how usually laidback he is.