The Mermaid Queen Ending is an ending achievable during the Return to Mermaid Caves event. To access it, the player must complete the rebellion sidequests, defeat the Mermaid Princess, and finally accept Salenia's offer to become the new queen.
To the shock of everyone else, Eric(a) decides to accept the crown since they felt responsible for what happens next in these caves. The moment the crown is placed on Eric(a)'s head, torrents of memories from previous queens come flooding in, which will take them some time to get adjusted to. For now, however, they must escort the rest of the Cutie Knights back on land and report what has happened to Marianne.
The news shocked even Marianne, and while Eric(a)'s teammates were happy for them at first, they soon realise their friend will be a mermaid until their rule is over due to the power of the crown... and since a mermaid queen's rule only ends in death, the transformation into Erica the mermaid queen is effectively permanent. While Marianne is quick to cheer the girls up, she's also quick to establish relations with the mermaids, ensuring that Erica won't be completely separated from her friends on land. Finally, after saying goodbye to one of the Cutie Knights - which depends on existing route lock or otherwise highest affection - and promising to throw a feast once the remaining three graduate, the new queen returns to her kingdom.
Despite what she said, Erica ends up getting accustomed to her new royal life much faster than she expected thanks to the crown, though unfortunately it erases Wendy from her mind as well. Time quickly flows, and Erica becomes more befitting a queen with every passing day while still keeping memories of her old life intact. While Veronica and Isabelle unfortunately couldn't attend the graduation feast, Erica still has a great time with Natalie, Alice, and Daisy, the new replacement and mascot of the team, and also learns that Natalie is doing wonderfully as the team's leader.
Depending on the character Erica has the highest affection with, the ending then details different outcomes.
- With Natalie, she always comes to help should the Cutie Knights find themselves engaged with a particularly nasty aquatic foe.
- With Veronica, Erica would regularly receive gifts but eventually Veronica has to move on and thus the gifts stop.
- Alice, having no home to return to, decides after graduating to join Erica in the mermaid queendom and gains some notable popularity due to her talent as a magical singer.
Regardless, Erica, with the help of Marianne, would later bring her people to a new world and found Wendy's Rest, but she never tells a soul about the significance of the name, much to the frustration of later historians. Much, much later still, when the realm faces a new darkest hour, she would return and... that is a story for another time.