Alright, so there is surprisingly less reviews for Blue Exorcist the manga than I expected. Seeing as most reviews came from people who've read up to chapter 30 or 40, years ago, I decided to give this a try.

I'm one of those people who've watched the anime first and I watched it a while ago. I tried the manga afterwards but only recently got invested into reading the whole thing. So this is my opinion after reading up to chapter 88 of the manga. I will try to spoil as little as possible in this review, so I'll have to leave off ranting about some of my favorite characters in here.

I'll start with personal enjoyment. I personally loved this manga. I thought the anime was decent, but didn't expect much out of the manga. Yes, some characters annoyed me and the narrative slows down in some places. Yes there were several cliches in here. Overall though, I'd give it a 9 for enjoyment because after I got into it, I couldn't stop till I've read all the chapters. That was around chapter 50 I think.

The story starts off pretty much a cliche. Parent figure dies, MC swears to become stronger and beat up what killed the parent figure. Drama between siblings (and this still persists.) and MC goes to a magical school to become stronger! While this seems like the generic shounen, the plot gradually thickens. A lot of stuff comes to light in the recent chapters about the Blue Night and demons. It's not as simple as I thought it was. While still retaining some lighthearted parts, the manga gets a lot darker after the anime veers off.
I'm giving the story 8/10 because it still had a pretty generic beginning and the rescue the damsel trope does get repeated a bit which gets annoying.

The art is really good. It's usually very crisp and detailed. A lot of the characters are distinct. The colored spreads are also beautiful. It portrays disgusting stuff as... very disgusting (You'll see what I mean when you get there). The only complaint I have about the art is how sketchy it gets during some action scenes. I'm not sure if the author ran out of time or if it's done on purpose. Either way, being the only bad thing I found of the art, I'm giving this section a 9/10.

Getting to the final section of this review, the characters! There are many lovable characters in Blue Exorcist. The main character, Okumura Rin, is kind of the generic shounen MC, but sometimes proves to have more insight and depth than you'd expect. He cares a lot about his twin, Okumura Yukio, but part of the conflict of the series stems from the tension between them. You can say that Rin wants to surpass Yukio, but Yukio sees Rin as the stronger one and has an inferiority complex in a way, that is aggravated by the different characters he meets later on. They are both lovable in their own ways though, and it is hard to believe that the MC is the older twin because of how hyper and childish he acts compared to the more solemn and composed Yukio.

Minor character-wise, there are quite a few female characters in the manga, though male still takes the majority. Out of these female characters, I'd say Shura is my favorite because she is pretty badass. The females have the potential to be strong, but as previous reviews say, there are characters like Shiemi that can be strong but acts weak and always has to get saved by people. She does grow a bit more bold later on, but I still don't like her very much. Izumo is the tsundere of the series and also annoys me, but not to a huge extent.
I am not sexist but for this manga, my favorite characters are all male. I like some of the more eccentric characters in the manga, from the Demon Headmaster Mephisto who holds the strings of many unexpected puppets in the manga, to the probably sociopathic Lightning who shows up a lot later in the series. I like the different world view the manga presents us, from the more naive students, to more cynical students, through the manipulation of Mephisto and the carefree view of some other characters...

For the characters, I would have given an 9, but the damsel in distress thing puts it down to 8/10.

Overall, I think this series deserves a 9 for the ratings. There is still room for improvement in the future and the plot of the story is still thick! Now if only it had more frequent updates than the current monthly ones...

Hope this helped!