Lillie made her anime debut in Alola to New Adventure! as a student at Melemele Island's Pokémon School. When Ash stumbled upon the school, Lillie tried in vain to warn him before he was trampled by her classmates' Tauros. She quickly demonstrated her fear of Pokémon when she cowered behind Ash upon being approached by one of the Tauros, only to be startled by his Pikachu. Later, when Ash described an unfamiliar Pokémon that he but no one else had witnessed, Lillie deduced that he saw Tapu Koko, the island's guardian deity.
In The Guardian's Challenge, after Ash enrolled in the school and showed his new classmates the Z-Ring he received from Tapu Koko, Lillie explained the guardian's fickle and mischievous nature to him. She and the other classmates saw Tapu Koko for themselves when it appeared to challenge Ash and Pikachu to a Pokémon battle, during which Lillie advised Ash about Tapu Koko's techniques.
Lillie is a polite and kindhearted girl who is concerned with the well-being of others. She is terrified of physically interacting with Pokémon, and tends to freeze up whenever one Pokémon enters her personal space; in SM005, she goes so far as to wear a full suit of diving gear to avoid making contact with Pokémon during a fishing class. When other characters point out her fear, she denies it by saying she simply chooses to avoid Pokémon.
Despite her fear, Lillie enjoys studying Pokémon from a researcher's perspective, and is comfortable with being around them at a safe distance. She is extremely knowledgeable about the behavior, types, and background of Alola's native Pokémon species and regional variants, including guardian deities such as Tapu Koko. However, she appears to know little about Pokémon in other regions, as seen when she expresses surprise that Exeggutor—a Grass/Dragon-type in Alola—is a Grass/Psychic-type elsewhere.