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.
At the end of noughts And Crosses, Sephy and Callum confessing to each other that they love each other as he is hanged.
The eventual reconciliation between Sephy and Callie at the end of Checkmate is truly beautiful.
Moral Event Horizon: Jude's murder of Kara. Possibly he himself considers it to be this, as he is notably more heartless afterwards, and much as he suppresses his conscience, it's clear that he never forgives himself for it.
Narm Charm: When Jude tells Cara that nought women often try to look like Cross women. He has a point, but then he tells her that they can buy panties which are padded so they can have more curvy hips, it just becomes funny. But still sad.
Much of Callie Rose's childhood counts as this, to the point that she is talked into becoming a suicide bomber while still a child. It is chillingly believable that she could be driven to this by the hell of her upbringing, and the manipulations of her uncle.
What Do You Mean, It's Not for Kids??: The series is targeted at Young ADULTS, and should not be read by any one under 13. Still does not stop parents buying them for a 10 year old then complaining.