Just about every time someone surprises Kaoru, or he has an Oh Crap moment. He's like a deer caught in headlights. Or a squirrel.
Like the time he blurted out that he finds Nana beautiful. SHIII-.
After the Nana's experience with the leather outfit in the first chapters she has a mock exam the next day, when the teacher tells her she's got the highest score she doesn't make the connection until the teacher asks how she relaxed... which causes Nana's whole face to blush.
When Nana wanders into a sex shop... while trying to give Kaoru chocolate... mmm-hmm.
Nana nobly volunteering to 'help' 2-F at the event in chapter 50 by putting make-up on Kaoru the Playboy bunny, while he's tied to a chair. Her amusement is ours also.
Nana is invited over by Kaoru's mother because he's making homemade udon. They walk in on Kaoru treading the dough and muttering "...boobies...boobies...". His mother's like, 'WELP'. (It makes sense in context.)
In chapter 62 Nana spaces out wondering if Kaoru has something to do with everyone becoming more considerate towards her during running practice when she's at the blocks. When she tries to start, her legs push off but the rest of her doesn't move, causing her to hit her head on the track and flip onto her back. Furthermore, the sound effect when she flips has a love heart beside it.
In the same chapter, Kaoru asks Tachi for help in English, but in return she makes him promise to turn up and cheer on at the athletics meet...
Tachi: C'mon, Kaoru. It's called "give and tail"!
Kaoru: Give and...?
Tachi:(acting superior) Don't you know that, Kaoru? It's "give and tail"!
In Black Label, Sarashina-sensei suggests that Tachibana tease Kaoru, thinking that he will get some entertainment out of it. Not only does he get what he wants, but a part of him seems rather jealous that she is doing this and paces about as he regains composure. Kaoru on the other hand is flustered like crazy, pretty much shaking in her hands while something else also rises (he finds this out later). Oh and Nana... she can't think of anything to say. Yes, the expressions of all of this is just priceless.
God bless the "Dog Girl" arc which combines this and Crowning Moment of Heartwarming. Chapter 73 features Nana and Tachi playing tug-o-war and it is absolutely adorable.
Speaking of which, one of Kaoru's two friends discovers he has an orgasm-detecting dog. And they go over to Kaoru's house to test it out. When they discover he's been *gasp* studying, they wonder what happened to their friend.
What makes this funnier is them discovering that Kaoru - despite his obvious Character Development - reacts exactly the same as his friends have before, which his friends react joyously to.
In Black Label, after Tachibana and Sarashina-sensei have left for Tokyo, leaving Nana and Kaoru alone at the house, we get this exchange in reference to Nana's Lethal Chef qualities:
Nana: W-why do you keep doing this? Am I not even allowed to set foot in the kitchen?
The farcical "The Crepe Truck On The Hill" arc is full o' funny. Kaoru, sick and in bed, goes off on a bit of a bender when Tachi teases the poor boy with crepes that Nana bought for him, goads him about his feelings for Nana and pesters him for another breather. She leaves, goes into the kitchen... then he hears his door open again and just goes off on a tirade that snarkily ends with, "Go buy some more crepes. But no panties". Problem: he was expecting Tachi in the room when he heard the door open and went off, not Nana. So upon hearing this, Nana leaves the room, takes off her panties and then leaves his house to go buy some crepes from a popular vendor on a hill in the middle of a public park on a windy day. When Tachi walks in a bit later to check on Kaoru, they discover that Kaoru has, unintentionally and unknowingly, sent Nana off on a "breather"... which leads into things.
Tachi's first reaction upon discovering this is to assume that this IS a wanted breather and tease. By attempting to take off her panties to join in the "fun". Kaoru is far from amused, if flustered.
Upon getting to the park, Nana discovers that she has walked into a pervert's playground; what with her not having undergarments, the wind being rather strong and all the boys waiting at the bottom for girls to come down the stairs for a sight of panty shots. Red seems to be a very nice colour this year.
After that Nana bumps into Yuuki Haruka (captain of the track team at Tachi's school), who saves her from taking that stair case and they get to talking note Correction - Yuuki does most of the talking mostly about boys being perverted, Tachi's racing performance, Tachi and this guy in her life, Kaoru. All the while heaping praise upon praise onto Nana, who doesn't think that she deserves any of this because she is just as much of a pervert herself. Seriously... rainbows are straighter than Yuuki.
Nana is overcome by her emotions for Kaworu and walks toward the railing where the boys are looking for upskirt shots. Yuuki desperately clings to to an oblivious Nana's waist to keep her skirt from being blown up, and ends up exposing her own underwear to passers-by on the same level.
And then Nana drops her cash on the floor by accident, only to be saved by Tachi who slides in and lands on her ass. No, I am not making that up.
After a brief chat... there is a problem. Tachi was supposed to be here with panties, namely Nana's own - she is empty-handed.
The topper on the whole farce is when Kaoru turns up anyway, just after Nana has had an epiphany regarding Kaoru. Her overflowing feelings cannot be contained and she runs towards him (having not noticed Yuuki at any point). Meanwhile he's trying and coughing and failing to shout at her to keep her bloody skirt down. As she descends the very stairs she agonised over earlier, he gives up and rushes up them, grabbing her legs at just the wrong (right) angle for him to unbalance Nana to have her fall, crotch-first, into his face, knocking him out as his head meets pavement. You can't take 'em anywhere, can you?
Porn-expert slacker meets under-the-thumb overachiever in the nail-biting search for one man's precious porn:
Hiroshi: And uh, when do you find time to study?
Doumoto: You and me, we ain't ever gonna be friends.
In chapter 99 Nana is out with girlfriends. One is angsting over one of their relatives' impending marriage, saying an engagement ring is like collaring yourself to someone, and the marriage to an S&M slave contract, causing Nana to become lost in her thoughts and increasingly turned on.
Yukari: But that lewd fantasy is exciting too. <3
Ai: Jeez, Yukari you pervert.
Yukari: C'mon Nana, you understand, right? That excitement, right?
When Kaoru has Nana bound in a way that's almost complete sensory deprivation, she thinks of her predicament, and thinks "Well, I guess it can't be helped". Then she freaks out a little at how casually she thought that.