idk if i am right but, kagami wrote cat16 (and probably wra19 too) when his writing was not influenced by the bl ships but despite the fact kagami wrote gureshin in a rather "more than just friends" way. like im not disrespecting any ship but the fact that shusaku and narumi dynamic was also written in a rather sibbling like. okay but talk abt gureshin NEVER once did kagami try to put a sibbling relationship. next is out of all his friends' death kagami made shinya's the most hurting for guren, to a level that guren say that if 'he becomes angry at mahiru then shinya will be sad' and that 'there was no meaning of causing the catastrophe if shinya died.' kagami could have used a female chracter for that, also ik he goes on alwayss commenting how maahiru is a godess etc, etc BUT he makes guren more inclined towards shinya's views. infact he shows in wra19 how guren kind of despised mahiru whereas was sooo deemed to save 'shinya and the others'. and then all that heartbeat, meaning of life talks and hugging for 1 hr scences i mean-
not to mention while shinya was always shown to be heavily admired to guren especially with that "meaning of life was meeting you" and how shinya grew up surviving even killing others but when it cam to guren he risked is life more than once; guren was also shown to always think abt shinya's words, to a level that when his friends died he was able to keep his sanity only because shinya said so. and there is just SO MUCH MORE abt them. again not disrespecting any other ship, but i dont think any dynamic was written this well in the early times.
so, i dont know if i'm being too much delusional but like given kagami's love for boobs and women, i always wonder WHY would he want his fav chracterr (guren is his fave chararcter, no one can change my mind) or even if i dont consider him the fav chracter , but like one of the protagonist to have such a dynamic with a GUY?
not to mention the type of PERFECT women mahiru was shown to be kagami just went to make guren somewhat despise her and kinda made thier relation tocxic.
all these thing make me woner did kagami really wanted gureshin to a thing?, or kagami just wanted guren to have a harem with a guy in it? or was he planning to all that family kind of thing (they are too gay for family thing ngl). idk therefore i'm asking u.
DO U THINK EVEN IF KAGAMI WAS NOT EFFECTED BY THE POPULARITY OF THE CHARACTERS AND THE BL SHIPS, GURESHIN WOULD HAVE BEEN CANON??
Thanks for the question, anon. God this brings back old shipping memories and I can't help it. I miss GureShin...
Nevertheless, about your question, I always say that Kagami wrote GureShin by accident. And by accident I mean that he never intended them to become this close. Friends, of course but both of them got a dynamic by themselves that Kagami probably didn't even expect.
But let me start from the beginning.
The first exhibit is the first interaction of Guren and Shinya in Vampire Reign. It's when Guren was sleeping in the meeting room and Shinya steps in to prevent him from trouble. Considering what "later" (in universe before) happened in Cat16 and Res19 this scene feels so out of character to me. But in my eyes it shows how Kagami thought Shinya's and Guren's friendship would develop into. And hell was he wrong. It gets better when Shinya appears a second time in Nagoya. But a lot of things happened already in Cat16 at that time (as both VR and Cat16 were written an parallel).
To keep it short, in the beginning Kagami didn't know where their friendship was going and most of it happened while writing Cat16.
This is underlined by his after words in the novels where he said he never intended Cat16 to turn out this long and the characters had much more to tell. I suspect Shinya was one of the characters that got a lot more screentime and character arc than intended and probably got a lot more chemistry with Guren than planned.
Writers that rely more on feeling than planning - so called pantser and Kagami is definitely one - often experience that their characters develop.a "will of their own". It's not like the author can't control their actions but the characters are so lively that ideas keep popping up a lot and the dialogue basically writes itself.
And I suspect this is what happened to Guren and Shinya (and most likely with the other characters of Cat16 as well). Their interactions started to generate more ideas and it felt right for the characters to become closer.
I don't think Shinya was intended to be part of the harem. He just happened to find more liking in Guren and Guren in him - sort of. And Kagami wrote along.
Also, Mahiru probably was never intended to end up with Guren (and hopefully never will). Guren rejected her from book one. She's a femme fatal and she's the antagonist of Cat16. She's meant to be attractive, addicting but evil and unpredictable. She isn't meant to stay but be the sweet danger she is.
Shinya happened to be the opposite. The steadiness with a friendship ideology Guren wanted to believe in. And not the power hungry path of Mahiru.
But your question is valid. Why would Kagami have one of his favorite characters have such a dynamic with a guy? When he already has a harem and a beautiful bitch that wants him.
I think he wrote it in a sense of "found family". Though the story this found family is always more important than romantic or sexual love.
You can be really close to someone without liking them romantically or sexually and probably Kagami writes Guren and Shinya as close guy friends. Not as siblings that's true, but just pals being their whole world for each other (*cough*)
Hell, why did he had to write them with so much sexual tension and banter between them? I always say, in the end it's up to every person if they want a relationship with someone they have a tension with. And in Guren's case neither him nor Kagami probably even considered the possibility. But in stories this kind of tension is difficult to ignore. The ship isn't coming out of nowhere. Though I feel that Kagami doesn't intended to ever make them canon as in admitting they have romantic feelings for each other.
Still, I always say they are and will stay canon because their (plantonic) feelings are still valid and very much canon. So that's undeniable.
But why all the romantic stuff if they aren't intended to be romantically intertwined? One point is that Kagami is intentionally fueling the ship with fan service. He knows fans ship it and whenever he has an idea, he will write it down because he knows it will have an impact on the fans. He doesn't have to, especially since he doesn't have to write Guren to be as fond of Shinya as he being the only one that he can fully trust. But yet here we are, with so much fan service that makes their romantic platonic feelings for each other valid canon.
In addition there is a lot of gay and bi erasure in Japanese culture. It's getting more accepted over time but it's not yet at a point where it is fully recognized in media and in everyday life. It's a wild guess and I can't say if it's true but I kind of suspect Kagami to not even considering that Guren and Shinya's relationship could be anything but a found family. Like haha, look at the fan service, not nah they can't be gay. They are both guys. They like women. At least Guren does... bi people exist... You know? Accepting to be bi can be a long process and accepting that a character that was never intended to bi could be into a guy could also take forever or be ignored forever. This is heavily speculative. But I always feel as of Shinya wants to tell Kagami he really likes Guren and Kagami is like "as a friend, right?" and then hell proceed writing fan service about them.
To sum it up, GureShin probably got "canon like" by accident. They never were intended to end up together and probably won't but as always, I'm still here for the gayness and I'm happy about every fan service crumb I'll get in the future.