An Oath to the Sea! Minami's Most Important Treasure! (海への誓い！みなみの大切な宝物！ Umi e no Chikai! Minami no Taisetsu na Takaramono!?) is the sixteenth episode of Go! Princess Pretty Cure.
- Air Date:May 11,2015
- Previous:Episode 15
- Next:Episode 17
- Opening:Miracle Go! Princess Pretty Cure
- Ending:Dreaming☆Princess Pretty Cure
- Minami's brother, Wataru, and her dolphin, Tina, appear for the first time.
- Cure Mermaid receives the Miracle Bubble Dress Up Key and uses Bubble Ripple for the first time.
- It was revealed that Minami's family owns a grand hotel known as the Kaido Grand Hotel and that Minami's family is part of the group called the Kaido Group.
The flashback shown that Minami drown and the dolphin saved her. In the present, the trio visit the seaside lodge owned by Minami's brother, Wataru and his group. Minami was so glad to see her brother and Haruka and Kirara didn't know that they were siblings. They went swimming under the sea but Haruka look afraid and Minami will escort her, her brother remind Haruka that Minami doesn't like swimming and the dolphin appear in front of Minami, who revealed to be Tina, the one who saved her when she is young. They having dish and sees Wataru enjoy himself with his friends. While Haruka and Kirara enjoy themselves at the beach, Kanata inform them via the Princess Lesson Pad except Minami that there's a Dress Up Key somewhere.
Twilight, who was angered of this after seeing Minami chatting with Wataru, and volunteered the two commanders to attack, which Lock agree and angering Shut. Minami and Wataru having a conversation about how Wataru run a business for his family. Lock appear before them and trapped Wataru dream of becoming a heir of his family and creates an Orca Zetsuborg. Minami transform along with the other but Shut creates an Octopus Zetsuborg from the octopus hawker. Mermaid tells Flora that she can handle the Orca Zetsuborg because she can swim so Flora and Twinkle handle the other Zetsuborg but they were trap by ensnaring them with its tentacles. Tina evacuates the other fishes while Mermaid fend off the Zetsuborg and was overwhelmed. Before it attack Mermaid, Tina rush to protect her, which injuring her and they fell into the cave. Mermaid was saddened after Tina get injured and shocked that she saved her for the second time when she was young and the fishes surround them and the bubbles flowing everywhere and Mermaid received a Miracle Bubble Dress Up Key and come out from the sea. Twilight become angry and order Lock to have Zetsuborg take the key and Mermaid perform Bubble Ripple at it along with Octopus Zetsuborg and free Flora and Twinkle before defeat them with Trinity Lumiere. Twilight become frustrated that Flora and Mermaid were just false princess before leaving along with the two commanders.
During afternoon, Wataru play with other kids and the Cures know that the sea are the treasures to Wataru's dream and Minami still remember Tina and how they stick together because the sea belong to her dream.
- Haruka Haruno/Cure Flora
- Minami Kaido/Cure Mermaid
- Kirara Amanogawa/Cure Twinkle
- Wataru Kaido
- Prince Kanata