Sarah Michelle Gellar Honors TV Dad Robin Williams on His Birthday: 'Smile, Though Your Heart Is Aching'

Nearly a year after Robin Williams’ tragic death, his onscreen daughter Sarah Michelle Gellar is honoring the iconic comedian for his birthday.

Gellar, 38, played his character’s child and business partner on CBS’s The Crazy Ones, which ran for one season in 2014. Williams would have been been 64 on Tuesday.

Inside Robin Williams's Private Struggles – And His Comedic Genius

Robin Williams could do it all: make an audience helpless with laughter, move them to tears, astound filmmakers with take after take of improvised comic genius.

“He was a guy who had the highest of highs while performing,” Bob Zmuda, the creator of the Comic Relief charity, tells PEOPLE. “But when he walked off that stage and went back to the dressing room alone, he could face the lowest of lows.”

Sarah Michelle Gellar Calls Robin Williams the 'Father I Always Dreamed of Having'

Robin Williams was not only a costar to Sarah Michelle Gellar in the CBS comedy The Crazy Ones – he was the dad she “always dreamed of having.”

In a statement to PEOPLE, Gellar reflects on Williams, who died of suicide Monday at the age of 63, and the indelible mark he made on her and her family. Gellar played the straightlaced daughter Sydney to Williams’ eccentric ad man Simon in the CBS comedy that was cancelled in May after only one season.

Sarah Michelle Gellar Shares Favorite Buffy Episodes with Internet

Sarah Michelle Gellar is currently starring in the CBS sitcom The Crazy Ones, but to many fans, she’ll always be Buffy Summers from the long-running WB series Buffy the Vampire Slayer.