The Mermaid Queen Ending is an ending which, during the Return to MErmaid Caves event, the player complete the rebellion sidequests, defeat the Mermaid Princess and accept Salenia's offer to become the 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 realize their friend will be a mermaid until their rule is over due to the power of the crown, and since the queen's rule only ends in death, the transformation is effectively permanent. While Marianne is quick to cheer the girls up, she's also quick to establish relations with the mermaids, ensuring that Eric(a) won't be completely seperated from her friends on land. Finally, after saying goodbye to Natalie and promising to throw a feast once the three graduates, the new queen returns to her kingdom.
Despite what she said to Natalie, Eric(a) ends up getting accustomed to her new royal life much faster than she expected thanks to the crown, though it also erases Wendy from her mind as well. Time quickly flows, and Eric(a) 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, Eric(a) 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.
Later on, Natalie ends up spending a lot more time with Eric(a) when she wasn't fighting Magicos, and their relationship blossoms thanks to Eric(a) now free of the camp's restrictions and her concern about her new gender, though due to her bloodline marrriage is obviously prohibited. Even when Natalie becomes one of Eric(a)'s most favoured concubines, neither of them ever felt as happy a when they were together.