When Elina, Hue and Bibble were travelling to the Wildering Wood, the were stopped by a group of Laverna's firebirds. Hue flew low so he could evade them, but he disturbed Nalu because he flew into a mermaid cove, Crystal Cove. Nalu was angry with Hue, but calmed down to speak to Elina.
Hue thought that the firebirds would not come into Crystal Cove, as they hate water, but he was mistaken. Nalu gave Elina, Hue and Bibble some magic seaweed to eat that allowed them to breathe underwater. Nalu guided the trio toward to Wildering Wood, and as they parted ways, Nalu told Elina he hoped to see her again. Nalu was not seen again in the movie after that moment, but Elina and her friends managed to defeat Laverna, and Elina got awarded wings.
Elina found out soon after she conquered Laverna that Nalu had been kidnapped by Laverna's fungi. He was being kept at the Crystal Cove, and was to be forced to lead Laverna and her fungi to the Immunity Berry; The Immunity Berry would allow anyone who ate it to become immune to all magic, and that was exactly what Laverna wanted. Nalu reluctantly agreed, as Max had threatened to destroy Mermaidia with poison. When Nalu had already left with the fungi, Elina arrived with Bibble. Even though she couldn't find Nalu, she met a stubborn mermaid named Nori who is a friend of Nalu's.
Nori and Nalu had planned to meet at the Crystal Cove, but Nori hadn't known Nalu had been kidnapped. Elina used magical seaweed to breathe underwater, and she and Nori went to search for Nalu. Nalu had sidetracked the fungi and led them astray to the Carousel of Confusion, a place with hypnotizing waters that make people forget what they're doing. Nalu directed them in circles to keep them confused for as long as he could, but they soon realized his trick. Meanwhile, Elina and Nori got to Mermaidia and found an oracle named Delphine so they could ask for help. Delphine told them to travel to the bottom of the Depths of Despair to find the Mirror of the Mist, which would lead them to Nalu.
Later when near a brackish swamp with a powerful whirlpool, Max asks Nalu the correct way to the berry. Nalu tells them they have to go through the whirlpool to get there. They were concerned about going through that whirlpool at first but when Laverna threatened Max to hang him by his toes and feed him to her Firebirds, he certainly agrees to go down there.
Delphine gave Elina a pearl necklace which would allow her to trade her wings for a tail; She needed a tail or she could not reach the Mirror of the Mist. If she stayed underwater for too long, she would stay a mermaid forever. Elina and Nori reached the Depths of Despair, but Nori got trapped by a sea plant. Elina used the necklace so she could swim as a mermaid and save Nori. They got to the Mirror of the Mist, and it made a bubble to lead them to Nalu. Nori and Elina followed the bubble and they locate magic fruits that can reveal the true self of whoever eats them. Nori and Elina went to the surface and found Nalu being guarded by fungi.
Bibble distracted the fungi by using a berry to sing opera, allowing Elina and Nori to free Nalu. He informed them that Fungus Maximus, the fungi leader, had found the cave where the Immunity Berry grows. Nalu, Nori, and Elina fought the fungi and got the berry that the Fungus Maximus had picked. Nori and Elina secretly swapped the berry with a fake one, and had the real berry. However, Elina had remained in the water for too long, and was stuck being a mermaid forever. Nori told Elina to eat the berry that would reveal her true self, and Elina was transformed back into a fairy, with wings.
With Laverna's plan ruined, Nalu saved and Elina a fairy again, Nalu says that everything ended perfectly. He held hands with Nori, which confused her because she thought Nalu loved Elina. Elina told Nori that she and Nalu are just friends, and that he was in love with Nori. Nori was embarrassed to reveal it, but she did let him know she returned his feelings.
Prince Nalu is a merman. He has brown hair and hazel eyes to match. His attire usually consists of a single sling of seaweed wrapped over his otherwise bare torso. He appears to be very muscular with a toned chest and well defined abdominal muscles. His merman tail was a beautiful deep sea-green color.
- The name "Nalu" is a male Hawaiian name meaning "wave."
- In Barbie: Fairytopia, Nalu was credited as "Prince Kai." The name Kai is also water-related, meaning "ocean" or "ocean wave" in Japanese.
- Despite being a prince, Nalu met and was friends with Nori, a commoner, and they later began to date.
- He is the first male character who always remains completely shirtless, Faban being a close second.
- Nalu's voice actor also voiced Prince Carlos, a character also from the Fairytopia series. Likewise, Nalu's love interest, Nori, was voiced by the same person who voiced Mariposa in Barbie: Mariposa and Her Butterfly Fairy Friends.