This is my version of Ariel the Magical Girl. Her skirt can transform into a mermaid tail, just like in Pichipichi Pitch.

Her story: She was a mermaid who longed to be a magical girl. She saw other magical girls fighting and wished she could be one, but only girls with legs could be magical girls. So she went to the sea witch and asked her to make her one, and so she did. But in return, the witch asked her to do evil deeds, though Ariel is so clumsy that she always ruins those very plans unintentionally. You could say Ariel is a magical girl who's half villain, but too clumsy to do either role well. She prefers to enjoy life and the beautiful things in it.

She doesn't have a secret identity; she's very absentminded and forgot to create one.