From my last book to this one, readers have kept questioning this point. Answering once wasn't enough, and neither was answering twice. If I didn't answer, the same questions would keep coming up, so I'm posting another standalone note to explain it.
In Japan, there are generally five forms of address.
First, addressing someone directly by name. This generally happens in two situations: one, you're very close and on affectionate terms; two, you're angry and about to fight, or you mean to provoke them. Of course, in the latter situation, people often use the full name, such as Naruto Uzumaki!! Sasuke Uchiha, and so on!
Second, san (romanized as san).
Third, chan (romanized as chan).
Fourth, sama (romanized as sama).
Fifth, kun (romanized as kun).
The first of these, san, is easy to understand. It's basically the polite suffix Japanese people use most often in everyday life. Generally speaking, if you aren't sure whether your relationship with the other person is close or distant, you can't go wrong with san, regardless of gender! Regardless of age or status, anyone can use it. To put it plainly, you can also use san for someone younger than you or below you in status, as a sign of basic respect.
The second, chan, is basically used between family members and friends who are very close. Take Sakura Haruno, a familiar example from Naruto: she's addressed as sakurachan. The sakura at the beginning is the romanized Japanese reading of Sakura's name, and adding chan afterward expresses affection. In Chinese, this is generally translated as "Little Sakura"!
The third, sama, isn't very common in modern Japan, though you'll see it more in the service industry. Readers who often watch Japanese anime and dramas should know that when you enter a shop in Japan, whatever the line of business, the staff will add sama to the end of your name to show respect.
Friends will sometimes use it jokingly, though. In Chinese, we translate it quite bluntly as "Lord/Lady So-and-so," meaning a person of distinguished status.
The fourth, kun, is the point people have kept complaining about in both this novel and my previous one. Readers keep insisting that kun can't be used for women. Let me tell you all clearly: in Japan, although kun is more commonly used for men, women can also be addressed with kun. It's just less common.
Generally speaking, this is a polite form of address. Of course, in Japan, women are most often addressed with kun by male teachers or older male superiors; it is indeed rare in other circumstances. So why do they do this? Here's a very simple example: imagine a fifty-year-old male superior addressing a twenty-year-old female subordinate. If they aren't close, calling her directly by name is ruled out first. Then, since he is so much older and higher in status, calling her san would feel strange and would also leave the female subordinate uneasy. Calling her chan would seem a little creepy and could suggest sexual harassment. In this situation, surname + kun is the best way to address her.
Readers who have watched some Japanese workplace dramas about women will certainly understand.
As for why the protagonist addresses Mei Terumi with kun in the story, first, it shows basic courtesy, and second, kun feels a little more distant than san. Kei wants to use this form of address to convey that he respects Mei Terumi but keeps his distance.
For readers who still don't believe me, seriously, just look it up on Bilibili or Baidu, or find someone who knows Japanese to explain it to you. I really haven't been making things up or lying to you.
In Japan, you really can address women with kun!