Finally reached the last chapter of Blue Exorcist and after dwelling on it for a while, I think I've figured out one of the biggest reasons it never became as popular as other major shounen manga (aside from the anime's inconsistent adaptation) and its because the main friend group doesn't do the story many favors?
Don't get me wrong I like Bon, Shima, Miwa, Shiemi and Izumo as characters and if they were in another series I probably wouldn't have said this, but in Blue Exorcist, I don't think their presence (except Shiemi, in the final arc at least) really add much to the story or elevate it in a meaningful way.
Bon started off great. I loved how he subverted the typical burly rival archetype by being intelligent, disciplined and how he really wanted to hate Rin, but just couldn't because deep down he knew that Rin was a good person. The problem is that after the Kyoto arc, his character doesn't really go anywhere. Even becoming Lightning's apprentice doesn't lead to much development. He mostly ends up serving as the audience surrogate and continuing his role as the group's straight man (not to mention that aside from Shiemi, he's the only one with nuanced relationship with the MC.)
Izumo has a lot of personality and would probably easily shine in a slice-of-life or romance anime, but her character arc feels VERY self-contained and mostly divorced from everything else going on. Matter of fact, once her arc concluded, she really doesn't have much narrative importance beside being Shiemi's sidekick.
Shima is the one I'm most conflicted about. He can be fun and the spy reveal had a lot of potential, but afterward he starts to feel more like a plot device than a character. The story uses him however it needs to move the plot forward, but we rarely get enough insight into his own thoughts, motivations, or goals beyond the thrill he gets from living as a double agent. It's a shame, because I think there was a lot more that could have been done if the story spent more time showing his PoV. I did enjoy his fight against Homare, too bad it's probably the last major thing he'll ever do in the manga.
And Miwa.........like why is he even here? Sorry for any Miwa's fan reading this, but he's easily the most insubstantial member of the main cast. At least the others had an endearing personality and entertaining interactions with others, Miwa on the other hand doesn't even have that.
Again, I don't hate any of these characters and I wouldn't call any of them "bad" characters (except maybe Miwa). My issue is that the MC's core friend group and inner circle should contribute a little more to the story than these characters do, especially considering how much panel-time they received, but looking back, I feel like Shiro/Yuri/Mephisto/Lighting/Shura/Yang/Osceola/Igor have left more of an impact than them with less panel-time.