Necrozma first appeared in a flashback in Ultra Origins!, where the Ultra Recon Squad recounted its history. It was shown to have stolen the light from the Ultra Megalopolis and was seeking to steal Alola's light as well.
Necrozma made its first present-day appearance in A Mission into Darkness! It was challenged by Lusamine and a reluctant Guzma, who wished to stop its plan to steal Alola's light. It battled Lusamine's Clefable and Nihilego, as well as Guzma's Golisopod, whom it defeated with ease and sent flying through an Ultra Wormhole.
It entered the wormhole itself in Necrozma's Shadow! to confront Lewis' Solgaleo and Nebby. It battled both, throwing Solgaleo from the Altar and pinning down Nebby. It then proceeded to absorb Nebby into its body, transforming into Dawn Wings Necrozma. With its new power, Necrozma opened several Ultra Wormholes across Alola, allowing Ultra Beasts to enter Alola. It then fled back to Ultra Megalopolis.
In the following episode, Necrozma battled Solgaleo once more and was able to hold its own against the Legendary Pokémon. Its control over Nebby was broken after Lillie reminded Nebby of their bond by performing Menacing Moonraze Maelstrom with it, damaging Necrozma greatly. Solgaleo and Lewis then attacked with Searing Sunraze Smasher, but Necrozma managed to absorb the attack's light and flee to the top of Megalo Tower, where it transformed into Ultra Necrozma.
Necrozma battled Solgaleo, Nebby, Aipom and Poipole atop Megalo Tower. Despite fighting five opponents on its own, Necrozma was able to dominate in the battle. However, after Poipole evolved into Naganadel, the tide began to turn and Necrozma was finally defeated by Solgaleo's Sunsteel Strike, reverting to its normal form.
In The Mother Beast of All Battles!, Necrozma returned to Alola to help stop the Ultra Beasts, having grown to respect Lewis and Lillie. It and Solgaleo willingly fused to become Dusk Mane Necrozma, boosting Solgaleo's power and giving the two the power to combat the Beasts. Eventually, the Ultra Beasts were returned to their own world and Necrozma separated from Solgaleo, returning to Ultra Megalopolis.
Necrozma is a Pokémon revered by the people of Ultra Megalopolis as The Blinding One. It is a Pokémon who was intent on getting what it wanted: light. To this end, it forced Nebby to fuse with it in order to satisfy its need for light. It is an incredibly powerful Pokémon, particularly as Ultra Necrozma, being capable of fighting multiple opponents at once without too much difficulty.
Necrozma's personality changed slightly after it was defeated by Lewis and Lillie. It grew to respect them and seemed to regret its previous actions, as it had caused the invasion of Alola by the Ultra Beasts. Necrozma then decided to make amends by helping the group drive the Ultra Beasts back, showing that it is not an inherently bad Pokémon.