This news comes via fellow voice actor Veronica Taylor (Ash Ketchum), who shared an announcement on social media, calling Lillis "an extraordinary talent" and "a bright light".
Lillis played a number of roles during her tenure with the Pokémon anime, including several of the creatures themselves, but is perhaps best known for her work in bringing Misty and Jessie to life.
It was confirmed earlier this year that Lillis had developed breast cancer, and Taylor referenced this by simply stating "Rachael was so thankful for all of the generous love and support that was given to her as she battled with cancer".
A memorial will be planned for Lillis at a later date. For now, we join thousands of Pokémon fans in thanking Lillis and remembering her remarkable contribution to the enduring anime. In addition to her appearances on TV, she also provided voiceover work for several movies, including Pokémon: The First Movie, Pokémon: The Movie 2000, and Pokémon 3: Spell of the Unown.
Her work providing the voice for Jigglypuff has also been utilized extensively in the Super Smash Bros. franchise.
Our thoughts go out to Rachael Lillis' family and friends at this difficult time.