Kiichi's antics are for the most part funny moments.
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 bank 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.
When on an 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.
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.