Nihilego first appeared in An Intruder in Paradise!, where it appeared through an Ultra Wormhole in the middle of Aether Paradise. It began to attack the conservation area, forcing Lewis and Mars to battle it. After a long battle, the two were able to send Nihilego back through the wormhole. Later, after the group has returned to Akala Island, Lusamine opens the wormhole again and Nihilego merges once more. Lusamine expresses her admiration of its power, before capturing Nihilego with a Beast Ball.
Nihilego returns in The Cracking Dimensions!, where Lusamine sends it out alongside her Clefable to battle Lewis. It proved itself to be exceptionally strong, almost defeating Aipom. The battle was called off when Nebby and Lewis' Cosmog were forced to open an Ultra Wormhole, allowing Lusamine and Guzma to escape to Necrozma's world.
Nihilego was used by Lusamine, alongside Clefable and Guzma's Golisopod, to battle Necrozma in Ultra Megalopolis. It fought well, but was overwhelmed by Necrozma's power and was defeated and sent through an Ultra Wormhole back to Alola.
At the conclusion of A Towering Battle of Legend!, Lusamine requested that Nihilego grant her the power to stop the Ultra Beasts' rampage. In response, Nihilego fused with its Trainer, transforming her into the Mother Beast. In the following episode, Nihilego, as the Mother Beast, helped to defeat the Ultra Beasts and send them back to their own worlds. After this was accomplished, Nihilego separated itself from Lusamine.
In The Calm After the Storm!, Nihilego was released by Lusamine, who decided it was better off living with its own kind. Nihilego then entered a wormhole created by Solgaleo and Nebby and returned to Ultra Deep Sea.
Not much of Nihilego's personality was shown, although it appeared to hold similar ideals to Lusamine, which is what drew it to her and caused it to allow itself to be captured. In battle, Nihilego is a brutal attacker, often attacking without warning, making it a dangerous opponent.