Kambal Sirena

Filipino fantasy TV series Kambal Sirena (Twin Mermaids) features Louise delos Reyes as twins Perlas and Alona. The pair are daughters of a forbidden love affair between Marissa, a human woman, and Damos, a merman prince; that love affair ended badly when Damos was killed by Enrique, Marissa's jealous former fiance, when the twins were babies. Though both twins are comfortable in the water, Perlas is human-shaped except for the gills she has behind her ears, while Alona is a full-tailed mermaid.

As children (Hershey Garcia playing both) the twins loved each other, but Alona felt neglected because she had to be kept at home to hide her tail, unlike Perlas who could go outside and attend school. Due to a series of misunderstandings Perlas, Marissa and their human family believe that Alona has been killed, but in actuality Alona has fled to the ocean to be raised by her paternal grandmother, Queen Arowana of Sirenadia.

The twins grow up with very different lives. Perlas is overprotected by her family and lives in constant fear of her gills being seen, while Alona is raised as a princess and learns magic. Then love gets thrown in the mix: Perlas falls in love with Kevin, her boss at the sea park she works at, and has to contend with Kevin's vicious ex-girlfriend, plus the dramatic coincidence that Kevin's mother is Enrique's sister, who has never stopped blaming Marissa for Enrique's death. Meanwhile, Alona deals with the responsibilities of being Arowana's heir, but also befriends Jun, a kind human man who helps protect her from villagers who want to capture her. The twins' reunion looms on the horizon in a twist on the Hans Christian Andersen story, where Alona rescues Kevin from drowning, then swims off and has to watch Perlas "take credit" for the rescue when she wakes Kevin up on the shore.

Like most teleseryes/telenovelas, it's a high melodrama story with some expected twists and turns in its 78-episode run, though there's less twin-swapping/mistaken identity shenanigans than you'd think. There are brief underwater scenes interspersed through the show, straightforwardly filmed and looking great for TV, and these are used as set-dressing for simple studio-filmed composite scenes of Sirenadia that allow the merfolk to speak underwater.