Unmarked spoilers are ahead, per Spoilers Off rule.
Kiichi's antics are for the most part funny moments.
Upon being formally introduced to Naoya, Kiichi proceeds to quickly reveal every detail of Reiji’s personal life to the young man, including the fact that he owns a Host Club right down to the type of partner Reiji is looking for. Reiji can only scream his brother’s name in frustration.
When he realizes Reiji and Naoya hadn't had a Relationship Upgrade yet, he comes to the conclusion that Reiji is impotent. Reiji doesn't find this funny.
Kiichi deciding on his own that Naoya needed to become a "bride of the Aoe house" and set off to buy him an engagement ring, much to Naoya and Haruomi's horrors. Of course, he shows he'd been teasing (maybe?) by buying Naoya a watch instead.
At the beginning of the jewelry store robbery crisis in volume 6, Kiichi has an inner monologue about the situation at hand and thinks to himself how sloppy and amateurish the robbers are. Then one points his gun at Kiichi and snaps "We can hear you, you know!", revealing that Kiichi was having his inner monologue out loud.
Kiichi plans a surprise party for Reiji’s birthday, only for the latter to choose to work overnight on his birthday. The brothers proceed to get into a hilarious shouting match with each other over the phone while Naoya and Haruomi, who are setting up decorations for the party, watch on in silence. In retaliation Kiichi announces that he’s going to take a bath with Naoya and drags the latter off, leaving Reiji screaming in outrage on the other end while Haruomi sheepishly apologizes to him.
Adding to the humor of the scene is Kiichi holding a pair of Groucho Marx glasses while he’s arguing with Reiji.
The Fairy Tale motifs that occur during volume 7 with some of Kiichi’s interactions with Naoya. When Kiichi tricks Naoya with a box of spiked chocolates, the scene is presented as Naoya as Snow White accepting a “poisoned apple” from the evil queen (Kiichi). And when Naoya goes to see Reiji at the Blue Boy to deliver his birthday present, Kiichi sending him off is presented as Naoya as Little Red Riding Hood and Kiichi warning him about the “big bad wolf” (Jin).
When on a double date, Izumi gets fed up with Reiji repeatedly talking down to him and demands that Takamiya beat him at a game of pool. Takamiya isn't interested...until Izumi promises to do whatever he wants if he wins. Takamiya immediately snaps to attention afterwards. Though it turns into a Heartwarming Moment when Reiji gives Takamiya an easy shot so he can actually take use of that.
When a fight between Ian and Rin breaks one of the windows at Blue Boy, an extremely irritated Reiji orders them to clean ALL of the windows on one floor. When Ian protests because he’s a paying client, Reiji literally screams the both of them out of his office.
During their cleaning session Jin happens upon them, and when he started cuddling Rin Ian flies into a rage—and another window gets broken.
The fateful day that Reiji met Takamiya and Shiki, his lifelong friend and first love respectively. It involved a cross-dressing Shiki stealing money from Reiji, Reiji losing his taxi, Reiji stepping in dog shit, Reiji losing his hotel reservation, and finally Reiji being physically attacked by Takamiya's ex-boyfriend on the mistaken assumption that Reiji was Shiki.
Similarly, when Shiki and Takamiya find out when Reiji's birthday is they wake him up at three in the morning to have his birthday party, and then make Reiji clean up afterwards.
