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Nebby first appeared in The Decision and the Tournament of Six, where it escaped from Lillie's bag and ran off to Mahalo Trail, where it was attacked by a group of wild Spearow. Lillie was too scared to help it, so Moon stepped in to save Nebby in her place. When she got to Nebby, it retaliated against the Spearow by releasing a shockwave, causing the bridge they were standing on to break apart underneath them. Moon and Nebby were rescued from falling in the water by Tapu Koko, who caught the two and brought them back to safe land.
In Flash and Cosmog's Secret, Professor Burnet accidentally caused Lillie to recall painful memories of a traumatic experience caused by her mother, Lusamine. Lillie's distress caused Nebby to fly out of her bag and begin clawing at the sky, nearly creating an Ultra Wormhole. After Lillie fainted, Nebby stopped and went down to check on her out of concern.
In PASM17, after Sun completed his second trial at the Thrifty Megamart, he thanked the Rotom Pokédex for its help in the battle where it gave information about Tapu Bulu. To Sun and Lillie's surprised, Rotom revealed it learned about Tapu Bulu from Nebby. Hoping that Rotom could translate Nebby's speech and speed up his job to deliver Mirage Berries to each of Alola's guardian deities, Sun tried to get Nebby to come out. To their shock, Nebby emerged from Lillie's bag in the form of the motionless, cocoon-like Cosmoem.
Later, after arriving on Poni Island, Lillie and Gladion took Nebby with them as they traveled to the Poni Altar. A Xurkitree attacked the duo as they passed through Poni Wilds, forcing Gladion to battle it with Silvally. During the clash, Nebby flew out of Gladion's grasp and into the air, where it encountered another Cosmoem. The two Protostar Pokémon flew towards the Altar. Upon arriving, the guardian deities took Sun and Moon and placed them on the Altar with the Sun and Moon Flutes. By playing the flutes together, Sun and Moon caused the Altar to react and release a light that hit both Cosmoem.
The light caused Nebby and the other Cosmoem to evolve into Lunala and Solgaleo, respectively. With its newly-gained power, Lunala put Moon on its head and helped fend off the Ultra Beasts that had arrived to attack the Poni Altar. Shortly after, a larger than usual Ultra Wormhole appeared at the Altar with a hand emerging from it. The hand took control of Solgaleo's body. The possessed Solgaleo grabbed Sun with its tail and jumped into the wormhole. Lunala activated its third eye and flew into the wormhole with Moon still on its head.
Some time after arriving in Ultra Space, Lunala got separated from Moon and Sun. At some point, it came in contact with the Ultra Recon Squad, who used Lunala to traverse through Ultra Space via Ultra Warp Ride. At the Ultra Deep Sea, Lunala reunited with Sun and Moon, who had been trapped in the area for six months. After agreeing to work together, Sun, Moon, and Zossie rode on Lunala's back so they could return to Alola. Dulse stayed behind as Lunala couldn't carry more than three people on it. Zossie taught Sun how to use Ultra Warp Ride and told him and Moon about the various worlds the Ultra Beasts come from. After passing through an Ultra Wormhole, the three arrived at the Ultra Recon Squad's home world, the Ultra Megalopolis.
Nebby merged with Necrozma
Shortly after landing, a Stakataka appeared, hoping to battle with Necrozma. After it captured Sun, Stakataka attacked Moon, but she was protected by Lunala. Moon commanded Lunala to battle against Stakataka, but it was unable to use its full strength without risking Sun's safety. By firing an arrow at the Zygarde Cube in Sun's bag, Moon released several ZygardeCells that stuck onto Stakataka's eyes, blinding it and allowing Lunala to easily finish it off. The battle alerted Necrozma, who released Solgaleo and leapt from the Megalo Tower to try and absorb Lunala as well. Lunala tried fighting back, but was overpowered. Sun and Moon sent out their Pokémon to help, but were stopped by Lusamine. Eventually, Lunala was absorbed by Necrozma, causing the latter to become its Dawn Wings form.
From 星 hoshi (star), 雲 gumo (cloud), and 星雲 seiun (nebula)
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