I recently re-discovered my love for She-Ra: Princess of Power, the highly dated but very pretty television spin-off from its masculine counterpart He-Man and the Masters of the Universe. She-Ra is the beautiful heroine of the show, a princess who gains fabulous powers thanks to her magic sword. She goes around Etheria fighting in The Great Rebellion against the tyrannical Hordak, hoping to one day gain freedom for its people. In The Great Rebellion, She-Ra gangs up with various beautiful characters (who are obviously designed for an audience of young girls), one of whom is a mermaid that appears in a few episodes of the series.
Mermista the princess of an underwater kingdom called Salineas. She's the only member of her people who joins She-Ra in the fight against Hordak, and she's a useful ally due to her underwater abilities. She can also gain legs whenever she wants, which is also an advantage.