To keep her secret, the mermaid could have a few trusted friends who help her. They could cover for her when she has to disappear suddenly (maybe to go to the ocean). And she would be very careful about her emotions. Since mermaids might be more emotional than humans, she has to control herself so that she doesn't give away her true self when she's around her crush.
The Secret Garden Mermaid Story may start with a mermaid discovering the secret garden. As she explores it, she finds that the garden has a special power that can grant wishes. However, there are rules to using this power. She then has to make a choice whether to use the power for her own desires or to protect the garden and its harmony. Along the way, she meets other magical creatures that live in or near the garden, and together they face challenges and adventures.
I'm not sure specifically as I haven't read it. But it might be about a mermaid's hidden knowledge, perhaps her special powers, her relationship with the underwater world or with humans.
She could use magic. For instance, she might use a spell to make her tail disappear and look like a normal human. Also, she would need to be careful not to get wet in public, because if she does, her mermaid form might be revealed.
The mermaid is surely a main character. There might also be some friendly sea creatures like a talking fish or a wise old turtle that help the mermaid in the story.
I recommend 'The Strongest Immortal Martial Great Emperor' to you. The male protagonist, Ye Bai, was the number one genius in the cultivation world on Earth. After transmigrating to the Martial Artist Continent, he slowly discovered that he was the Demon King. The female lead was a transmigrator who was not introduced in detail, and the plot was mainly based on the male lead. I hope you like this fairy's recommendation. Muah ~😗
Well, in a mermaid secret high school crush love story, perhaps the mermaid has to hide her true identity from her crush. She might be worried that if he finds out, he'll be scared. But their love could be so strong that it overcomes all these obstacles. For example, they could meet by the school's pool where she secretly shows her mermaid tail to him, and he accepts her for who she is.
Maybe it's about a mermaid who disguises herself as a human to attend high school and falls in love with a human student there. For example, she has to keep her mermaid identity a secret while dealing with her growing feelings for this boy.
Ursula as a mermaid in fanfiction might also explore her backstory more deeply. She could have been a mermaid princess in a far - off part of the ocean. But due to a power struggle or a curse, she lost her status and became the villain we know. This could add a lot of depth to her character and make the fanfiction really engaging.