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The 25-year-old tweeted a quote from Antoine De Saint-Exupery's "The Little Prince" to her 105,000 followers:
Williams' last post on social media before his death was a #TBT happy birthday wish to Zelda.
"#tbt and Happy Birthday to Ms. Zelda Rae Williams!" he wrote in the caption. "Quarter of a century old today but always my baby girl. Happy Birthday @zeldawilliams Love you!"
The father-daughter duo, who were especially close, starred in a Nintendo commercial together in 2011.
In the commercial, Zelda asks her dad if he's confusing her with the fictional video game character. He smiles back, "Hard to say, you're both pretty magical." Watch it below:
Zelda - the firstborn of Williams and his second wife, Marsha - often accompanied her father on the red carpet.
Zelda was her father's date to the "Happy Feet Two" premiere in 2011, along with Williams' third wife, Susan Schneider.
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It always looked like Zelda and her dad were having fun together. Kevin Winter/Getty Images
In 2009, Robin Williams kept a tight hold on his daughter at the premire of "World's Greatest Dad" in Los Angeles. Kevin Winter/Getty Images
Here Zelda poses with her boyfriend, mom Marsha, and dad at the premiere of "License to Wed" in 2007. Kevin Winter/Getty Images
In 2006, they both appeared on MTV's "TRL."
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Williams leaves behind three children: Zelda, Cody, and Zachary.