Solgaleo was one of two Cosmog obtained by the Aether Foundation for the purpose of opening Ultra Wormholes. It was given to Guzma to open Ultra Wormholes in order to summon Ultra Beasts.
Solgaleo first appeared in Confusion and Monsters from Another World, where Guzma used the Nebula Pokémon to open multiple Ultra Wormholes to summon Ultra Beasts to attack the Trial Captains who came to negotiate with Team Skull at Po Town. During the battle, a Nihilego dragged Guzma into one of the Ultra Wormhole, separating him from Cosmog.
In Battle in Vast Poni Canyon, it was revealed that Cosmog had evolved into a Cosmoem and was picked up by Plumeria. Plumeria tried using it to force Faba to tell her where Guzma was taken to. Before she could get her answers, it flew out of her hands and met with Nebby in the skies above Poni Island. Upon arriving, the guardian deities took Sun and Moon and placed them on the Poni 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 Cosmoem and Nebby to evolve into Solgaleo and Lunala, respectively. With its newly-gained power, Solgaleo attempted to link its aura with Sun's to fend off the Ultra Beasts that had arrived to attack the Poni Altar, but Sun rejected the Sunne Pokémon to instead focus on Faba. Shortly after, a larger than usual Ultra Wormhole appeared at the Altar with a Necrozma's 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.
After being taken into the Ultra Wormhole, Solgaleo was fused with Necrozma to become Dusk Mane Necrozma. Necrozma then traveled to Ultra Megalopolis, where it attempted to absorb energy from Solgaleo to regain its true strength on top of the Megalo Tower. Six months later, Lunala arrived in Ultra Megalopolis with Sun and Moon and battled a Stakataka that had entered the city. The battle alerted Necrozma, who released Solgaleo and leapt from the Megalo Tower to try and absorb Lunala as well.
In Regeneration!! The Power of the Sun and the Moon!, Solgaleo absorbed the energy from the sun in Alola that came through the Ultra Wormhole that Necrozma had created. It jumped through the Wormhole and appeared at Mount Lanakila, where it teamed up with Sun to fight Ultra Necrozma. Linking with his aura, Solgaleo attacked the Prism Pokémon with multiple Sunsteel Strikes, but was quickly overpowered. When Necrozma was about to finish off Sun and Solgaleo, Zygarde attacked the Prism Pokémon. Shortly after, Moon arrived and gave Solgaleo a Max Revive while her Decidueye stopped Necrozma's movements with Spirit Shackle. With Necrozma's movements stopped, Solgaleo teamed up with Lunala and Zygarde and defeated the Prism Pokémon.
Several months later, Solgaleo took Gladion and Dulse into Ultra Space to look for Faba.