Barbie: Fairytopia is an early entry in the CGI film Barbie canon, and the first in the series to be an original story instead of an adaptation of a preexisting work. In it, Barbie plays Elina, a fairy without wings who is kind, gentle and brave, though she is teased by some of the other fairies for her disability. The story is kicked off when strange are afoot in Fairytopia: a guardian is kidnapped and many fairies start falling sick and losing their ability to fly. Thus Elina and her trusty sidekick Bibble head off on an adventure to get help and eventually save the day.
Among the places that Elina visits during her travels is Mermaidia. She first meets Prince Nalu, a merman, and he helps her avoid her enemies by safely traveling underwater through Mermaidia, which Elina is able to do when she eats special seaweed that allows her to breathe in water. It's a brief sequence in Elina's journey.
We get to see a lot more of Mermaidia's underwater realm in the sequel, Barbie Fairytopia: Mermaidia. Prince Nalu has been kidnapped by Laverna's minions, and Elina is recruited to help him. But before she can help him, she has to find him, and that investigation takes her into the underwater realm of Mermaidia, and into contention-turned-friendship with Nori, a mermaid who is in love with Nalu and suspicious of non-merfolk. Elina even temporarily transforms from a fairy into a mermaid in order to help her on her quest.
The majority of the film takes place underwater, and accordingly the world of Mermaidia is more elaborate compared to the brief glimpse in Barbie: Fairytopia, which only came out a year earlier! But it's still an early entry in the Barbie series, and making it a noticeable starting point to how much more detailed the franchise's later underwater worlds will be. Nori is a great new character to interact with Elina, with a fun design right off the bat, and the arc of her friendship with Elina is a fun one.