This book was so engaging, the storytelling is phenomenal. The characters all felt so real. They were all well thought out and had detailed histories, personalities, motivations. The two main characters, Tavia and Effie, both felt like unique characters who had complex personal stories. Even though it was told from two points of view, neither felt like it was better or worse and they didn't overshadow each other. Each of the main characters had a story that was interesting to learn about in different ways. 
Tavia is going through life with a dangerous secret, she's a siren. She has to see her people be discriminated against and oppressed because of the power that they hold. The connection between the sirens struggle and the real-life struggle of poc, especially black women, adds so much to the story. Not to mention the implications that only black women can be sirens. The way she is treated by those who know who she is is difficult to read but really draws that comparison. Her parents try to stifle her because they are afraid that she could get hurt, others paint her as dangerous, or they try to out her to prove they are better than her. Tavia's journey from wanting to get rid of her power to

taking control of it and using it to save people, meanwhile giving sirens a good name, possibly turning things around for the better overall.

shows just how powerful a black girl can be. Tavia not only fits to stay safe every day, but she also protects her sister.
Effie doesn't know who she is. Her mother died when she was young and she never knew her father. She has lived with Tavia's family for a few years and they are like sisters. Effie is a self-conscious, shy, anxious girl who is only comfortable when she is a mermaid in the Ren fair. She is particularly embarrassed by her unexplainable skin condition that causes her to itch at her dry skin, especially when she is nervous or upset. Effie is Tavia's everything, and Tavia is hers. Effie starts to have issues when she begins to have regular blackouts and strange visions and hearing. She begins to think there may be things her grandmother isn't telling her. Effie goes through a lot of character development in this book. She starts off as a shy girl who doesn't know who she is,

and becomes a powerful gorgon living with her father and her boyfriend.

It was so great to see these girls stand up to those who have put them down.
My one issue with this book was the pacing. There is a mystery element to this book, so obviously most of the climactic events happened right at the end, but I think that they could've come just a little earlier and still had the same impact. The end felt rushed and lacked the detail that the rest of the book had. Especially the last chapter, which basically just wrapped everything up. That part just felt like the end of a movie when there is a voice-over and a random montage of moments that happened after the main events of the film. I wish we would have been given just a little more. I think even just having a chapter from both perspectives instead of just one would've improved it overall. 
Overall, a highly enjoyable book that had powerful messaging and great characterization.