Kaoru doing routine maintenance on the toys. To be honest, any time Kaoru shows his knowledge about S&M in order to take care of Nana during or after their breathers (for example, he immediately shoots down using a whip on Nana for the spanking because he realizes it'll be too overwhelming for her and thus wins Tachibana's advice on how to do proceed properly). He makes Nana's feelings are first on his priority list before doing anything too off-the-rails.
The train ride into the country that kicks off ch.3 of Black Label. It's a lovely little playful, friendshippy moment. It quickly turns funny as they realise their separate trips have the same destination.
Black Label has been doing this a lot. In chapter 18, there is a odd but touching moment between Nana, Tachibana, Sarashina and Kaoru. After watching Kaoru tie Tachibana for a few moments, and listening to Sarashina talk about 'rope drunkenness', Nana reminisces about the first time that she herself was bound (using the same rope that Kaoru is using currently) and comes to a realization... that rope, and Kaoru by extension, is hers and she wants it on no-one else but her. This brings her to faint, and Kaoru stops everything to rush to her side and tend to her. Tachibana and Sarashina simply look on at the collapse of the session, noting that Kaoru and Nana are no-longer Twice Shy about their emotions to each other... just like they are, they don't need to push the relationship any farther.
The end of Chapter 58. It also counts as a Tear Jerker.
It says something about this manga that Nana barking at Kaoru is the single most heartwarming (and loving) moment in the story so far. Kaoru ended the pet-play breather when Nana had to wet herself. Kaoru left Nana back in her room, berating himself for forgetting to account for toilet breaks. Nana actually came back, divined his constant nervousness, and 'asked' for the continuation of the breather because she didn't think he had anything to apologise for. Additionally, she wants them as much as he does.
Next chapter, Tachi and Nana stay the night and the next day. When Tachi kisses Kaoru on the cheek in thanks, he freaks out because Nana's right there. So Tachi restrains him to let Nana give him her kiss on the cheek.
More or less all of Black Label chapter 29. Nana and Kaoru simply holding hands is imbued with more lust and affection than all of Black Label, because of the history they have together.
When Kaoru finally coaxes forth Nana's true feelings about her parents and friends at school hiding underneath her fake smile. It's a major development for both of them, because not only did Kaoru intuit what she wanted and needed, Nana trusted him enough to break her defences, and was secure enough that she could let it all out in a full-blown, primal scream freak out. At the end of the breather, he takes her snotty, drooling, tear-stained face in his hands and says even now, she's beautiful. MY HEART. To her face, even, when before he considered it a relationship-wrecking slip.
In chapter 96 we have porn bros.
In chapter 100, Tachibana and Nana get to talking over collars in Nana's search to replace the one that was lost. When Tachibana presents a similar looking collar, the second that Nana puts it on she can tell it's not the same. And there is a reason for this.. You see Kaoru has not only taken good care of the leather, he's modified the collar to the point where nothing else can replace it. A point that Tachibana seems to understand (in a way that only a submissive or a dominant can understand), at which point Nana seems to just break down emotionally... Yes Nana, "You have been very loved." And for once, Nana can tell.
In chapter 24 of Black Label, when Sarashina finaly pushes past his insecurities and declares, publicly no less, that he is never going to let go of Mitsuko. Being let go was the thing that they were both most afraid of.