Lillie first appeared in The Homecoming Crown!, living with Mohn in the Crown Tundra. In a series of flashbacks, it was shown that when Mohn wound up at the Ultra Deep Sea, Lillie rescued him and took him back to his own world through an Ultra Wormhole. It then used its powers to make Mohn see it as Lillie, and proceeded to live with him in a remote house for years. To avoid Mohn regaining his memories and leaving it, Lillie removed all the mirrors in the house and dirtied the windows, preventing him from seeing his own reflection.
Years later, Mohn's real family found Mohn in the Crown Tundra, unable to remember his real relatives. Lillie tried to prevent them from revealing the truth to Mohn, but ultimately, they ended up being grateful towards it for having rescued Mohn. Even once Mohn had regained his memories, the family still invited Lillie to stay with them and join the family as a second Lillie. It accepted the invitation and was caught by Gladion.
Unlike most Nihilego in the anime, Lillie is shown to be very caring, as seen when it rescued Mohn and took care of him during the time they lived in Galar. It is also shown to be very protective, being ready to attack Lillie and Lusamine when it felt they were trying to separate it from Mohn.
Lillie is also shown to have a lonely nature, as it didn't want Mohn to regain his memories, fearing that Mohn would abandon it if he did. In Helping the Hometown Hero!, it showed an interest in battling.