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.
Ensemble Dark Horse: Hideyuki had been created specifically to die, so his death would provide a reason for Ryo and Kaori's partnership. The author was quite surprised discovering how popular he was right after killing him off, and brought him back in flashbacks and implied he was the man Saeko loved, choosing him over Ryo.
"Funny Aneurysm" Moment: During a story Ryo gets a truly idiotic and half-asleep face, and, according to an old friend of him, it meant he was practically undefeatable, and the last time Ryo had that look he wiped out an enemy platoon in half an hour alone. It became really funny when Ryo's Roaring Rampage of Revenge for the villains having indirectly made him impotent for a while devastates their HQ, but becomes much harsher what had happened the previous time he had that face: he had been drugged with Angel Dust, leading to Ryo massacring an enemy platoon in horrible ways.
Ryo's fear of flying on airplanes, and even of airplanes in general, caused quite a few laughs at Ryo's expenses, even having Umibozu scaring Ryo with a toy airplane. Then Kaori remembered that Ryo is the only survivor of a plane crash...
Genius Bonus: If you know firearms you can understand many particulars before they're explained. For example, Silver Fox return may be guessed before his identity is revealed if you recognize the Remington Model 700 used by the mysterious villain to shoot Kaori's shoe and remember that he used a rifle chambered for the .308 Winchester round in his debut (the Remington Model 700 may use various rounds, but the .308 Winchester and his military variant 7.62x51mm NATO are the most famous chamberings), and serious opponents can be recognized by their pistols and revolvers (if they use anything bigger than the .357 Magnum round and are not Umibozu or fighting him, they're idiots) and how they use them (if they don't follow safety rules, they're idiots).
Germans Love David Hasselhoff: Tsukasa Hojo is often noted for doing such series. City Hunter is very popular in Europe, Latin America, Hong Kong and especially Taiwan, but was only a mild hit in Japan. In the US, its fame remain limited as well, mostly due to being an older series whose Anime adaptation wasn't dubbed until recently.
Les Yay: This occasionally occurs such as when Saeko did once hit on Kaori, but keep in mind that Saeko didn't know that Kaori is a woman at the time.
It runs in the female side of the family, as shown by Saeko's younger sisters. Reika, the middle, has no qualms at admitting she uses men all the time, and in her debut arc she succeeded in getting Ryo to be her temporary secretary for free by using a tiny wound he caused her while saving her life. The younger, Yuka, is a successful writer of crime novels at sixteen, and uses her sisters as 'inspiration'.