Hailey, a year-round resident of the island community, is Jill's cousin. She and her mother Susan are hosts to Jill and Meredith during their vacation, and always-enthusiastic Hailey can't wait to show Meredith around the island during her first visit. She has a great fondness for all things mermaid-related, and for texting acronyms. O-M-G!

Lorelei, Marina's best friend and constant companion, is an adventurous spirit who loves to venture about on land. When Marina is forced to trade places with Meredith, Lorelei finds herself protector to Meredith--and Marina's tail.

Jill is Hailey's cousin and Meredith's best friend. A drama student, Jill hopes to be an actress some day, and her improvisation skills might come in handy when the girls' summer doesn't end up going as scripted.

Barney is a naive, eager, talkative, and sometimes clingy merboy. After inadvertently learning Meredith's true identity, can he become a friend who will keep her secret?

Jeff, Jill's athletic older brother, has just graduated from high school, and turns up unannounced at Hailey's house for the Fourth of July. He knows Jill's friend Meredith--and he knows that the pretty blonde isn't Meredith--so he could complicate the girls' plans.

Susan is Hailey's mother, Jill's aunt, and a dear friend of Meredith's mother. She's looking forward to meeting Meredith for the first time and opening up her home to her--or whomever it is that Hailey and Jill have been harboring there.

Finn is Lorelei's father and the advance scout for the school of mermaids. He has been Marina's guardian since she was an infant. Though he is willing to help Meredith change back, he commands that she wait until the next full moon.

The Mean Girl of the sea, Calliope is the daughter of the all-powerful leader of the mer school. Armed with her loyal followers, there's nothing Calliope would like better than to cause trouble for Marina--or any of her friends.

Will is in the summer internship program at the aquarium along with Meredith. Though he can rattle off the scientific names of all the animals there, he lacks experience and initially shies away from getting his hands wet. Maybe with some expert tutelage, he can become better prepared for further study in marine biology.

Brittany is the third intern at the aquarium. Often with a disparaging remark for Will and Meredith, she doesn’t show any interest or knowledge regarding the animals in the aquarium or marine biology in general. She often breaks the rules and turns up in unexpected locations in the aquarium, but what is she rebelling against?

Dr. Hatcher is the marine biologist that supervises the interns at the aquarium. He is friendly and well-respected in his field, and he genuinely cares about all ocean life. He’s impressed by Meredith’s performance in the program, but how would he react if he knew why she’s that way?

Ray is the leader of the school of mermaids and Calliope’s father. Strict and imposing, he is suspicious of Hailey’s presence coinciding with Marina’s absence—especially since he seems to hold a grudge against orange-tailed mers.

Isabel is the eccentric and mysterious shopkeeper at The Mermaid’s Lagoon. She sells mermaid souvenirs and possesses vast knowledge of mermaid folklore. She may also have the ability to read people’s thoughts and emotions by looking in their eyes.