Aqua Regina is the goddess of the sea who oversees and rules over all mermaids, the Mermaid Princesses, and other sea creatures. She is an ancient and powerful being whose age is described as the Age of the Sea itself. In appearance, she has long blonde hair and blue eyes, which make her bear a strong resemblance to the main character Lucia Nanami. She wears a light blue crown, blue jewelry with red gems, a see-through shawl, and a large, puffy white dress, and she carries a staff adorned with a large blue gem, wings, vines, and leaves.

Aqua Regina is characterized by her kindness toward all mermaids, and she is known to be brave, having performed legendary actions to keep the ocean peaceful. She serves as a benevolent advisor and guardian, though her ability to act directly is severely limited. Thousands of years ago, she expended a great deal of her energy to seal away the Panthalassa, a clan of sea monsters that threatened to engulf the world in storms. As a result of using this immense power, she is unable to manifest fully on her own and typically appears only as a ghost-like form or phantom. Her primary role in the story is to empower the Mermaid Princesses. She gives them new songs and upgrades for their gear, such as bands and enhanced microphones, which increase their singing power and transform their outfits to be more glamorous. The only way to summon her completely is to gather the seven pearls belonging to the princesses of the seven seas.

The villain Gaito seeks to capture the mermaid princesses and collect their pearls to summon Aqua Regina for his own destructive purposes, intending to use her power to destroy the world. Due to her depleted state, Aqua Regina cannot directly intervene in conflicts, a limitation that forces the Mermaid Princesses to fight their own battles. She can only appear when the princesses are in grave danger to offer advice and bestow new powers before fading away again. Her presence in the sea world is absolute, and most marine creatures address her with the utmost respect.

Aqua Regina views the Mermaid Princesses as her daughters. Her relationships are primarily defined by this maternal guardianship, and her actions are all directed toward helping them complete their missions. In the manga, her role extends further when she saves Lucia from the brink of death and passes on her position, making Lucia her successor. It is later revealed that the name Aqua Regina itself is a hereditary title given to the goddess of the sea. Her development throughout the story is thus tied to the transfer of her divine responsibilities to the next generation, ensuring the continued protection of the ocean realm. Her notable abilities include creating new songs for the princesses, upgrading their magical equipment, and offering prophetic advice, all of which are essential for defeating powerful enemies.