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
Alas, Poor Rival: Nanami in chapter 46.1. Manabu finally realizes that Nanami's not a grade-schooler and so turns down her feelings. The worst part is of how you knew it would end like this — hell, the manga even lampshades it humorously that she had no chance ... and yet you really can't help but feel for Nanami because of how absolutely crushed she is.
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.
Manabu: It's made from a legendary material that was a gift from the gods and yet, it doesn't kill people.
Honor-Related Abuse: This is what the treatment of Manabu by his mother comes down to. Given that though his entire life, she never even gave him a hug, or even smiled at him, there is no conceivable way for them to form any relationship, since they never had any. Made more egregious by the fact, that his existence that displeases her so much, is as a result of her own indiscretion, while she was engaged to a man that was not his father. As if he was supposed to be take responsibility for her actions. Shiratori seems to agree with that sentiment, given his choice of words: "I don't want to say anything bad about your family or your mom, but..."
Inverted as well, in that this is treated more as a personal issue between Manabu and Azami, with the rest of the family seeming to stay out of it.
Insane Troll Logic: Manabu's uncle blames Reina, for the fact that he will choose her over his mother and his family. Nevermind the fact that his mom doesn't want to have anything to with Manabu - which is why "going back" involves being adopted by his uncle. And, there is the fact that, as he was so mistreated before Azuma moved out with him, there is no conceivable reason why would Manabu even want to be part of his family again, even if Reina wasn't an issue. Manabu himself can easily list a number of reasons why it's a no-brainer choice, which includes being forced to change schools, give up all of his friends, and being forced into an Arranged Marriage.
Manabu and Mayami's uncle Aoi. He's incredibly rude, snobby, and has spent most of his time in the manga being a gigantic asshole to Reina just because she committed the "crime" of being Manabu's girlfriend. Even worse, it's shown that he hates his own sister for an incredibly petty reason: it's only just because she overshadowed him despite him being the older sibling. That being said, his very first appearance in the manga is him grieving over the death of his wife Misaki, aka Kuroko and a flashback chapter shows that as much as he hated Azami, he was supportive of her relationship with Manabu's father despite the latter being a commoner, and felt really bad when the helpers showed him that Manabu's father was getting mail from another woman.
While it will be undoubtedly debatable if she deserves the sympathy, Azami counts too. While her abusive treatment towards Manabu is unforgivable there remains the fact that she broke down in tears when Reina told her that Manabu still loves her despite what she did to him and the fact that she fell in love with someone who was a total womanizer while she had an arranged marriage lined up for her, made even more tragic when we get to see how she fell in love in Chapter 61. The flashback chapter also shows that she wasn't always the terrible person that she was today; in fact she was on Manabu's level in terms of niceness.
Les Yay: Manabu's little sister Mayami is a masochist who enjoys getting beaten up by Reina.
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.