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Pokémon Ultra Sun
Dulse, along with Zossie, is encountered several times throughout the player's adventure in Alola, where they are investigating how Necrozma can be defeated. In learning about Pokémon battles, Dulse challenges the player to several battles throughout the course of the story.
Pokémon Ultra Moon
Dulse stays behind with Zossie in Ultra Megalopolis while Phyco and Soliera investigate Alola. He is first seen upon the player's arrival to Ultra Megalopolis, where he explains that Necrozma is at Megalo Tower.
After Necrozma is defeated, Dulse will gift a Poipole to the player in thanks.
Dulse debuted in Adventure in the Dark Dimension. He and Zossie arrived at the Ultra Deep Sea after riding on Lunala. Dulse had just left the Alola region after meeting with the Aether Foundation to create Poké Balls that could capture Ultra Beasts. Overhearing this fact, Sun and Moon approached the Ultra Recon Squad members and asked what their connection to the Aether Foundation was. After explaining themselves, Dulse and Zossie were asked by Moon if they could take her and Sun back to Alola. Though Dulse refused, Lunala went to Sun and Moon, as it was already acquainted with them. Having no choice, Dulse allowed Zossie to go with Sun and Moon to Alola while he stayed behind. Dulse attempted to call Phyco and tell him about losing Lunala, but he was only able to get in contact with Soliera.
Later, during its battle with Zygarde, Necrozma summoned Dulse from an Ultra Wormhole and pulled him towards its back. By absorbing the Ultranecrozium Z Dulse had in his possession, Necrozma was able to transform into Ultra Necrozma. The Ultra Recon Squad decided the best option to deal with Necrozma was to capture it, but they had no available Beast Balls due to Lusamine's meddling. Nanu revealed he could have another Beast Ball delivered, but told Sun and Zygarde to keep Necrozma busy while the waited for it to arrive.
Later, after Moon had joined in, Necrozma was finally taken down by a combined attack from Zygarde, Lunala, and Solgaleo. Anabel arrived with the final Beast Ball, only for it to be destroyed when Necrozma managed to launch one final attack. Sun revealed he picked up the Beast Ball Moon tried using on Lusamine earlier. Anabel gave the final Beast Ball to Zossie, who captured Necrozma in the hopes that the Ultra Recon Squad could potentially heal its injured state.
Several months later, Dulse used Solgaleo to transport Gladion through Ultra Space so he could rescue Faba, who had been dragged there by a group of Nihilego. Ryuki also chose to accompany them, much to Gladion's annoyance.
Dulse and Zossie used Lunala to travel through Ultra Space via Ultra Warp Ride. Due to Zossie's inexperience at piloting Lunala, she accidentally caused it to crash-land in the Ultra Deep Sea. After reuniting with Sun and Moon, Lunala left Dulse and Zossie to go back to its companions.