Lillie first appeared in JN111, living with Mohn, after having rescued him. It was shown through flashbacks, that Lillie used to live in the Ultra Deep Sea, where an Ultra Wormhole opened suddenly and brought Mohn there. It read Mohn's memories and saw his family and helped Mohn escape through another Wormhole, ending up in the Crown Tundra in Galar.
Lillie kept company to Mohn, who had developed amnesia and lost his memories of his real family, the entire time he stayed in Galar. Due to reading his memories, Lillie would often create illusions, usually disguised as Lillie and play the piano.
After Mohn reunited with his family and gained his memory back, Lillie attempted to leave, but was asked by Lillie, Gladion, Lusamine, and Mohn to join their family, which it happily accepted. Gladion then offered his Beast Ball and Lillie was caught.
Personality and characteristics
Lillie with Gladion's family
Unlike most Nihilego in the series, 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 since they weren't familiar.
Lillie is also shown to have a lonely nature as it didn't want Mohn to gain his memories, because it feared that Mohn would abandon it.
Lillie is the first:
Shiny Pokémon in the anime to be caught by a major rival.
Ultra Beast to be caught by someone other than a main character.
Pokémon caught by a rival onscreen outside of the series they debuted in.