Lillis, born in New York state in 1969, played Pokémon trainer Misty and Team Rocket antagonist Jessie, during the original series, which began in 1998.
She reprised the roles in Pokémon: The First Movie and Pokémon: The Movie 2000 along with other films in the popular Japanese anime series, which is also known for its games and trading cards.
In an Instagram story, the official Pokémon account said it was “deeply saddened” by Lillis’ death.
“Her performance in the Pokémon animated series will be cherished by the many fans who grew up with the characters she brought to life with her special talent.
“She will be remembered for generations to come, and we will always keep her close in our hearts. Our thoughts and condolences are With her family and friends.”
Her death on Saturday evening following treatment for breast cancer was announced earlier on Monday by fellow US voice actress and Pokémon star Veronica Taylor, who was the original voice of the show’s lead character Ash Ketchum.
“Rachael was an extraordinary talent, a bright light that shone through her voice whether speaking or singing,” she wrote.
“She will be forever remembered for the many animated roles she played, with her iconic performances as Pokémon’s Misty and Jessie being the most beloved.”
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Taylor added: “Rachael was so thankful for all the generous love and support that was given to her as she battled cancer. It truly made a positive difference.
“Her family also wishes to thank you as they take this time to grieve privately. A memorial is being planned for a future date.”
Taylor also wrote: “She filled our Saturday mornings and before/after school hours with her beautiful voice, her terrific comic timing, and her remarkable acting skills.
“I was lucky enough to know Rachael as a friend. She had unlimited kindness and compassion, even until the very end. She had a great sense of humour, was wonderful to be around, was incredibly intelligent, and had such a memory for moments – real or animated. She worked hard and cared deeply.
“I am not sure how this very dark void will be filled now that her light no longer shines in it. She passed away during the peak of the Perseid meteor shower.
“Like those shooting stars, Rachael was a magnificent wonder and truly special. She burned bright and vanished too soon. She will live on in our memories for eternity.”
Lillis also appeared in Teenage Mutant Ninja Turtles, Mobile Suit Gundam Unicorn, Guardian Of Darkness and Winx Club.
Her sister, who confirmed her age as 55 and not 46 as had been reported earlier, had previously started a GoFundMe page for Lillis’ medical bills, which Taylor had encouraged fans to donate to.