This book had quite the potential to be something really good. High school girls and occult stuff? Sign me in. Horror books are "my thing", and good horror books are hard to find; I'll give the writer that. And I'm not saying this book was a total failure or anything, but it wasn't that good as well. I have to agree that this was simply Mean Girls with someone throwing occult and demons and exorcism into a bowl, mixing it all together and hoping that something good would come out. 
 Basically it's a first person pov of a girl named Sofia (yay, that's me!) Flores, who just moved to a new city, and to a new high school. There she meets Brooklyn, the kind-of rebel, weird girl, and the group of popular, really religious girls : Riley, Alexis and Grace. Right away Riley "claims her" for her group of friends, and the rest of the story is Sofia being torn between gaining the approval of Riley and being on Brooklyn's side.Very Mean Girls style, I might actually call it a deja vu. Of course that Riley and her friends don't like Brooklyn and vice-versa and Riley is actually convinced that Brooklyn is either a demon or possessed by one. They decide to help a sister out and kidnapp her, so that they can perform an exorcism on her. You know, because what girl doesn't like to be exorcised any other day? 
 The writing itself is good, you can read the book nicely and quickly ( I read it in a couple of hours), but it has some issues. The girls talked to each other in the most annoying way possible, always bickering and being total bitches, and that gets to annoy me after a while. There is not that much of a background to the characters, in my opinion, and specially for Sofia. The part of the book that doesn't talk about the exorcism is kind of pointless because of the way it is written, in my opinion, and I would go well either without it, or with something more detailed and with a little bit more effort put into it. Because if you read the premise, you already know what it's going to happen. And the ending. Gosh, how I hate more-than-predictable endings. The horror part itself was really dull, to the point that there is no horror at all. There is only gore and blood, made to shock the readers, rather than to scare them. The parts that were supposed to be "the jumpscares" where laughable. If in an horror book, or movie, you can see things happening a mile away even before they happen, then it's not a jumpscare. 
 I had serious problems with the characters, and with the reason why they even tried to exorcise Brooklyn. Riley was the Queen Bee and the other ones were mindless sheeps following her, that didn't had any type of own will, or mind that told them that it was a really wrong thing to do what they were doing.Riley just wanted revenge because Brooklyn stole her boyfriend (ex-boyfriend, in fact, so Brooklyn didn't steal anyone. They are not friends, so Girl Code is not applicable here), and because she is probably psycho and thinks that believing in God absolves her from all her crimes. Alexis and Grace didn't do much through the whole book, and Sofia was just another girl who tried to be passed on as some kind of flawed hero, but failed. She was just annoying. They were all annoying. The only character I even liked was Brooklyn. 
 There were some other things that bothered me / I didn't understood, like what the hell was that between Sofia and Charlie? Why was Sofia so obssessed with the Quetzalcoalt ? How did Riley and Brooklyn ended up hating each other? Why didn't Brooklyn did what she did in the end, in front of Sofia, while she was being tortured? (I don't want to spoil anyone so I tried to put this question in the most vague way I could). The Sofia and Charlie romance thing was just to had some romance to the story, but what happened? Do they stay together? Do they like each other, although they abrely talked to each other? Who knows. 
 Final Thoughts : Teenagers are psychos fulled by hormones, specially teenage girls. 
 The Merciless 
 is a book that failed at delivering horror, because it more about gore, used to shock people. It was nothing special, and the premise sounds way more appealing than what the book really is, although it also spoils half of the book. I would not recommend this book as an horror book. It doesn't mean you shouldn't read it, or that I didn't enjoyed it, but as far as the horror goes, it's a poor book.