This review contains spoilers.
In their afterword, Kaito says that they wanted Blue Flag to be three things: about choices, to show the world as the characters see it, and to be a story where characters actually talk to each other, because in their experience as a romance reader, most problems could be solved that way. I think it's safe to say that they succeeded on all fronts, and not just because I feel brilliant for having figured out that how Taichi was drawn depended on whose eyes we were seeing him through. While this does have a happy ending, it also never stops being about the choices people made and continue to make, from Taichi deciding on his life's path to Toma and Futaba figuring out what they want from and for him. And while some of those choices may turn out to be a surprise, like Masumi's in the final chapter, which takes place seven years in the future, if you think back into the context of the characters and the story they do all end up making sense. For example, Masumi's choice ends up being more about her never having thought about life after Futaba – she was so focused on just one person and basing her purported happiness on that one person that she never managed to see beyond that to the other options she might have had. While it does feel like a bit of a cop-out to have her married to a man seven years after high school, her husband's words make it clear that she's bisexual, an area of LGBTQIA+ representation that gets less realistic focus in fiction. But even if that wasn't the case, what we can really take from Masumi's future is that she finally moved on from Futaba, something she was previously unable to do.
It's interesting that “moving on from Futaba” is actually something that both Masumi and Taichi, as well as Futaba herself, have to do. There are some implications that no one, including Futaba, really sees her for who she is. In Futaba's case that may be because she has almost no self-esteem, viewing herself as weak and fundamentally flawed. She's constantly looking for her reflection in someone else's eyes, whether that's the unquestioning friendship with Masumi or the blind devotion of and to Taichi. A piece of her knows that this isn't the best way to move forward, but she's so stuck inside her own shell that she can't do anything else, which is what she tries to tell Toma when she yells at him for confessing to Taichi. Futaba is afraid of losing her friends and her boyfriend because that means that she'll be forced to see herself as herself – and she's not sure that she can handle that. It makes her a bit of a nonentity as a character up through the end, more catalyst than individual. That's too bad, but it also may be because she can't see herself as the protagonist of her own story; in her mind she's always going to be the supporting character, something that seems to hold true even at her own wedding.
The two most striking moments in the book belong to Taichi. The first is when he thinks he's lost Toma after he stops coming to school, which causes him to destroy his bedroom in an explosion of emotions that even he doesn't fully understand. This is important not just because of the implication of the final chapter and bonus chapter (that he and Toma are married), but also because it underscores how much deeper his feelings for Toma run than even he realizes. Given the reaction at school to Toma's sexual orientation, if Taichi were bisexual, he's unlikely to have been comfortable even thinking about it given that he would also be attracted to girls, and the scene in his bedroom speaks volumes about what he may have been pushing deep down in his soul. It's also the first truly major emotional outburst we've seen from him, and that he can't even verbalize to himself what he's doing also feels significant. It carries over to his second striking scene, when he realizes that he might have started dating Futaba because if he could help her change, then maybe he could change himself as well. Rather than being about emotions he can't handle yet, this one is more a statement of his attempts to “normalize,” something he puts to Toma as causing him to not want to hang out with his best friend because it reminded him of how different he was from Toma and his other pals. Both add up to something he can't fully grasp until at least two years after high school – because that's how life works sometimes. Happy endings and graduation are so very rarely tied together like the stories say, and Taichi needs some time to grow up, just like everyone.
High school is not really the place where most people figure out who they are and how to be that person. That may be the lesson everyone has to learn, in the end – all choices influence each other, but the really big ones don't need to be made right away. Among the other messages in Blue Flag, that feels like one that stands out, because there's a lot of life after high school and change is inevitable. Sometimes that works out in your favor, sometimes not. But for Taichi, Toma, Futaba, and Masumi, it's just another bend in the road.