Ariel herself is well-written. It follows suit with other Little Mermaid things from Disney. I quite like Ariel BEFORE the events of the movie when she gives up everything for some guy she doesn't know. They're really going to have to work hard to convince me live action Ariel isn't even more irresponsible than the cartoon one, because now she's not only giving up her family, her world and her very self, but she's also stepping away from the kingdom she's protecting. Being a good protector matters to her in this book, so yeah, they're going to have their work cut out for them to make her come up with reasons for what she does.
I loved all the sisters, though not right away. Their dynamic is primarily bickering and fighting. Indira and Caspia are close, but they don't really like anyone else except sometimes Ariel. No one really likes Mala except Ariel because Mala was the perfect one they had to live up to. Perla and Karina don't like each other. And Tamika hasn''t been seen since the day their mother died.
Perla stepped out right away as my favorite. I love her design best, so I'm glad I liked her character in this, too. She's the only one to help Ariel right away. She's kind, warm, funny, a bit fearful but honestly, they all are except Ariel.
Karina was next. She returned to her kingdom quickly after Mala's kidnapping because she was afraid. She and Perla mend fences and she joins the sleuthing. I got a sense of Karina being very caring.
Tamika was never shown in a bad light. She's simply traumatized by fear over what happened to their mother. I don't even think she dislikes Mala like the others do, though it's possible she once did. She's sweet and gentle.
Indira and Caspia can barely be separated. Indira is a bit of a joker, while Caspia is more prissy. Neither of them are rule breakers. It takes them the longest to come around and I didn't 100% get where I loved either, but I definitely liked them by the end.
Mala herself was barely there, so it's hard to get a good read on her, but I liked what I saw. She has an interesting secret that I won't spoil.
This book is set when Ariel turns fifteen, as that's when protectorship is granted. She does become protector at the end.
Each sister has her own MerSong, which is essentially a superpower.
Each sea has a protector and a resident, which is a sea monster. In this book, the sea monsters take the role of the humans in the movie. They're what Triton fears and what the rest of the seafolk fear.
The queen's death is different in this book from what it was in Eric's Guide to Merfolk. Not sure which story they'll go with for the movie, but they're both the fault of Ursula and humans, so it doesn't really matter.
The book has the origins of Flounder (who I liked a lot more in this world than in the original) and the grotto. Scuttle is referenced as a friend of Flounder but doesn't appear. Sebastian is there but barely.
So yeah, it's basically Ariel solving a mystery alongside Flounder and her sisters and it's awesome.