What is this Ruka conundrum I'm talking about, and why is it so important? To be blunt and to the point, Ruka has had zero character development in the direction of being okay without Kazuya, that and she has her meme heart condition. In other words, a built in self-destruct should the author ever decide to use it, anti-depressants and depressants already cause significant problems for people without a heart condition. For Ruka, her medication plus alcohol, or sleeping medication, paired with heart weakening stress from a break-up seem to be more than enough of an excuse for the author to kill Ruka with a heart attack. There are a few heartless people that are quite fine with Ruka dying hoping her death will remove a major obstacle between their ship, but in actuality a Ruka death will more than likely be the end of the Kazuya x Chizuru ship.
Why does Ruka's death spell the end of the Kazuya x Chizuru ship? Kazuya and Chizuru have both shown that they are kind hearted people, Kazuya often sacrificing his own himself for others happiness, Chizuru does the same but to a much lesser extent, however, so does Ruka (and Sumi). Despite Ruka being a source of annoyance for Chizuru, both Kazuya and Chizuru owe her a great deal of thanks due to her assistance with the filming project, even though she had something to gain from the filming trip also. Ruka sent Kazuya the script that would ultimately be selected for the film, played an indispensable part in the play, and has a lot of personal relationship time invested in Kazuya, as well as Chizuru. Ruka is also a topic often broached in conversation between Kazuya and Chizuru, meaning that she is on their minds somewhat frequently. Neither Kazuya, Chizuru, or Ruka want to hurt each other, just imagine the guilt that would be brought to bear on Kazuya and Chizuru after learning that they had a hand, and are somewhat responsible for the death of a person, even more damaging that they knew this person personally. They would also learn of Ruka's heart disease, further shattering Kazuya and Chizuru's relationship, as this would mean that Ruka hadn't willing to unlimitedly guilt trip Kazuya, but was doing only what was necessary to be close to him.
It would be difficult for Kazuya and Chizuru to build a relationship on the grave of another person, especially someone who had deep feeling for Kazuya, someone whose death they had a hand in. The guilt will surely drive the two apart, likely ending the ship once and for all. What would happen next? Mami, being the opportunist she is, would likely swoop in and claim the broken Kazuya, with a slight chance Sumi could rescue Kazuya from her grasp before he could make the grave mistake of tying the knot with Mami. Ultimately, a Kazuya and Mami relationship hurts all parties involved.
So, what if the heart attack doesn't kill Ruka?
Once again, Kazuya is willing to sacrifice himself to save someone else, and if he has to give up on the chance of a relationship with Chizuru to save Ruka's life, I feel that he will.
What leads me to believe that Ruka will suffer a health emergency should Kazuya break up with her?
Ruka's character has developed to be less toxic, and more selfless, but she's also grown more fond/closer to Kazuya. She's not willing to give up on Kazuya, that seems to off limits for her character. The author seems to have set Ruka up to be an all or nothing character, he developed her to be less toxic, but has fallen more in love with the MC. She's set up to have one of two fulfilling endings, either she ends up with Kazuya, aligning with the trend of the tragic underdog who makes a comeback, or she dies, fulfilling the role of the tragic character.
How do we avoid this outcome?
About 100+ more chapters of character development, and a decent portion of them featuring Ruka, though this task must be handled delicately to avoid ruining Ruka's character, as there nothing worse for story and merchandising like ruining a good character. Ruka's character is one of, if not the strongest in the series, ruining it would be a huge blow as Ruka is the series second most favorite.
What do I think Reiji should do?
If Reiji is a good business man, I think a Kazuya and Ruka pairing would be the most profitable and satisfying in the long run. Ruka and Kazuya don't seem to have much story potential outside of the main series, all the other character do, for example, see Sumi. Mami has great spin-off story potential, Chizuru has great spin-off story potential, but Ruka and Kazuya seem to be fairly fulfilled in this series.
How should this be done to avoid the fanbase giving up on the series?
Kazuya and Chizuru should date, and soon. Ruka should be dismissed, only to be brought right back to make Mami believe that Kazuya is no long seeing Chizuru, but that rather it is all a song and dance for Grandma. Kazuya should learn that the feeling he thought he had for Chizuru was not love, but rather a grand appreciating for a best-friend/confidant, something that Kazuya lacks in his life. Meanwhile, Kazuya has developed feeling for Ruka, but doesn't want to leave Chizuru out of consideration for her feeling. Kazuya and Chizuru's relationship grows stale, and Chizuru learns the truth about Kazuya's feelings. Chizuru releases Kazuya to pursue a relationship with Ruka. Now Mami must be dealt with, Kazuya must appease Mami to avoid her ruining the family business, and hurting his grandmother. Mami wants Kazuya to tell his family all the details about his various lies, basically humiliating himself before her. Kazuya does this, is forgiven by his grandmother, and accepts Ruka as the potential future wife, but still loves and values Chizuru. Mami, satisfied with Kazuya's humiliation, tenders her resignations, and leaves. A new series is launched, with Chizuru as the MC, so we can have more of her as a character, and less as a plot device, with her not getting sidelined to Kazuya all the time.