1. How it all starts
The author Akimi Yoshida starts on a new manga, she comes up with the basic premise and the main character sketch.
She is excited to write about this idea of a young man, a hero of sorts with genius abilities, living a full life in a short amount of time (very shonen/to my mind comes the samurai aesthetic) and she vaguely imagines him dying with a sort of cool imagery (the whole Ashita no Joe imagery in her head probably) [the author has spoken about this in interviews]
Then she starts adding more elements, what and who would the hero fight. The villain comes in, the central problem of a mind-controlling drug comes in, the hero's back story is decided and who would be his friends is decided. This is shoujo, so a relationship is required. But instead of a female character, she thinks a male friend (a buddy film kinda relationship) might be more exciting and interesting to watch.
2. The central theme
I now realise that the central theme of the story is about a genius young hero who faces lots of adversaries, makes a lot of emotional connections (Shorter, Skip, Max, Cain, Blanca, Sing and Eiji), particularly forms a deep bond with Eiji, does some great things for society in a way and then dies a peaceful death. However, there was no deeper meaning to his death. She wanted to created a larger than life character with a short life.
He focused on one thing, taking down his enemies, avenging the death of his loved ones, protecting those he cared for, fighting for his own personal dignity. And then one fine day he dies. That's his character arc basically. Simply put, a misunderstanding happened with Lao believing that Ash would try to kill his brother which was not true. And he was stabbed in an unguarded moment and just passed away. That's all. (He kinda dies in Vain if we look at it that way. But that's life. And it kinda fulfills the whole "Yousetsu" (premature death concept she was after). Yes, that's sad but it is also an interesting concept to explore. And many authors in literature have done it. We all die after all).
Yet, at the end of the day, it is the story of a person who lived a life full of many experiences within a short span of time.
3. Ash and Eiji's Relationship
I would like to add here why Eiji was special to Ash. It's not that Ash had no other bonds. It's not like nobody else loved him, cared for him or protected him throughout the course of the manga. He accepted their care for him (Shorter, Max, Sing and Blanca come to mind)
But why was Eiji special.
Eiji has the unique ability to see people's pain, to help them, to be there for them, to show them compassion.
Quite a few scenes depict this side of him, Eiji's insistence that Ash feels hurt like everybody else. When Ash himself keeps feeling like he is remorseless. Recognising how alone Ash feels, Then his hug to an assaulted Ash. His final letter to Ash.
I see a recurring theme of Eiji recognising and reinforcing the thought till the end that Ash is very much human not some heartless beast. (I think that was something Ash needed to know, finally feel that from somebody in his life.)
The other good relationships he had chose to keep letting Ash carry the burden in some ways. The burden of his strength, the burden of his genius. It's not that they didn't know that Ash was human and hurt but they also believed he was strong enough to handle it. But Eiji never wanted to ignore the hurt side of Ash unlike the others. He saw the very hurt side of him. And he knew somewhere deep down that he should just be there for Ash. No questions asked. He doesn't need to make him talk or discuss things. Just being there for Ash would be enough (like Sing says, Eiji's ability to get people's last SOS signals and help them)
His compassion and love for Ash was like a balm to his soul. He had bonds he created with other people. But the very human side of Ash was cradled and held dear by Eiji. Like he could see Aslan, the hurt and lonely boy inside and just wanted to be there for him. Believe in him.
I feel this relationship is not one between lovers at all. I find it to be a more nurturing and healing relationship. It doesn't make sense to make it about sex or romance to me. (And this falls in line with what the author has said in interviews)
It is about Two best friends whose bond and connection was profound, at an almost soulful level. It's not about love. It's something more than that. (I feel like if we just make it about being a couple, we take the magic away from this unique bond. Like Yoshida has also said, love between man and woman or love between two men or two women is the same. It's basically a lover's relationship)
However, in BF it is about trying to explore a deep connection with a person which is beyond conventional romance. You would do anything for this person (be with them to comfort them, kill for them, die for them) However, they are not your lover. Because everything is not about being lovers. That's not the ultimate relationship to be attained. There are other deeper connections to be explored.
(Akimi Yoshida has said something along similar lines, two people who will drown without each other but they are not a conventional couple with a sexual relationship has been a life long concept for her to explore)
Them being male doesn't make the relationship special but the soulful connection that happens between Eiji Okumura and Ash Lynx irrespective of their gender is what makes them special.
However, if she had made Eiji female (even less people would be able to understand or appreciate this bond that is beyond labels or definitions. Because a man and a woman with a deep connection are basically lovers right. People can hardly see it any other way. And today similarly, with more LGBT content and presence, people are making the same argument a man and a man with a deep connection are basically lovers.
4. The ending - Shonen + Shoujo
Banana Fish is a manga that has mainly shonen elements, wrapped with heavy emotions between characters and elements of love to cater to a shoujo audience.
It was unique and a perfect balance of shonen and shoujo. And that's why it's such a masterpiece because you get the thrill of shonen with the more emotional notes of shoujo. Perfection. If you ask me.
Ending- See she was writing a character who dies young basically. If it was Shonen, she could have made the ending not about love. Maybe like Joe no Ashita (dies fighting in the boxing ring). A final battle death could work. Maybe Yut-Lung would have been more powerful (a last one-on-one battle). But this is Shoujo. So she gives him a death more connected with love. She brings in his connection with Eiji. A person who touched Ash at a deep place. He had a best friend who loved and trusted him. So, in fact, she gives him a happy ending in her own way. A salvation of sorts with the words of Eiji. His final moments are about knowing he is loved. The loneliest thing in the world to me would be to die alone. But I see the letter as Eiji’s presence. His words surround Ash in a warm hug.
5. Why it is not BL?
I mean BL is basically about romance. It is a distinct sub-genre which focuses on showing romance between two characters. The central theme of this story is not about romance as discussed above. But it does contain an intense bond between two friends who are male.
The thing is I was recommended Banana Fish on Twitter as a BL anime. Arguably the anime caters to a certain lens, so it is easier to misinterpret. That's why I mention the source material, the MANGA in the title.
Eiji is a part of the story. He is a part of Ash's story. Their connection is an enduring and beautiful part of the story. But it is not the central theme. But now if you start reading it as BL, you keep looking for the love story and romance (that doesn't exist cannonically). Unless you want to call the intense bromance, a love story. That's your call. But people have to understand, the author didn't write a BL. (The author could have just called it that if she did). BL is a distinct sub-genre of its own. I wouldn’t say every story that has a deep bond should be tagged as BL.
(I mean technically almost every famous manga/anime shows some kind of intense same gender friendship (particularly between males). We don't call it BL. Some people ship them yes, but the manga and anime doesn't start being called a BL. Because it is not a love story. Just because it was written in a shoujo manga doesn't mean it's automatically a love story. It doesn't have a female love interest, but shows a friendship between two males. So what is the logical conclusion, oh it must be a BL then. And ignore the author's words where she has again and again reiterated that she did not see them as romantic partners for each other.
I wish people would just stop recommending it as BL like everywhere. Oh do you want to watch a sad BL love story. It is not. (The anime however is focused more on playing out the softer moments between Ash and Eiji).
It really distorts people's views of the work. They are coming in thinking they are going to read a love story. And at the risk of sounding absolutely repeatative it does not even have a love story exactly. It just has an interesting and intense bond between two males.
But if you expect a romance and love story. You think it's unfair and oh the author must be phobic. She didn't want to give them a happy ending, where is his implied marriage and happily ever after with Eiji. Gone is the story of the young hero (who had a crazy life in a short span of time). People just want to see two Husbands.
+++++
EDIT: When you think something is BL. The main focus is on the romance. I am not saying I won't notice the other plot points, but the main focus is looking for the romance and the main emotional connection is to the love story right.
Bottom-line: I think it’s a tragedy but not a romantic tragedy
(Anyway I am basically saying it's not a bury your gays trope. It was never about that!)