In The Name Of The Mermaid Princess summary:

Princess Mio is betrothed to Prince Chika. However, they've never met, and he doesn't know her secretshe's a mermaid! When her tutor, Yuri, takes her out of the castle to meet her subjects, Mio begins to accept her true self. But Yuri has a secret too And when Mio tells her father, the king, she wants to live life openly as a mermaid, he punishes her. How can she stay true to herself?