But to the family of Christopher Reeve, the actor and comedian will be remembered for the "simple, steadfast friendship" he unwaveringly offered for decades.
Williams, who died of an apparent suicide Monday at 63, befriended Reeve, who died in 2004 at 52, when the two were aspiring actors and roommates at Juilliard in the early '70s. The friendship endured as they found fame, and their bond only grew stronger after Reeve's 1995 horse-riding accident rendered him quadriplegic.
Now, Reeve's family is honoring his funny and loyal confidante, issuing an emotional statement exclusively to PEOPLE a day after his death.
"For Robin, our dad was 'Brother Reeve.' Robin was a light in our family for as long as we can remember. He and Dad made each other laugh, and they stood by each other to the end. Our hearts ache for his family."
A family source also tells how Williams lifted Reeve's spirits like no other shortly after the onetime Superman's devastating accident when the comic came into Reeve's hospital room dressed as a Russian proctologist.
"He knew how to make everyone laugh in a way that can lift their spirits and lift their souls," says the source.
In a statement to media, Reeve's family paid tribute to the inimitable actor, looking back at a friendship that began at Juilliard and "transformed into a brotherhood."
"The Reeve family, like many, will remember Robin for the humor, energy and brightness he brought into our lives through the screen. We will cherish him, though, for the quiet and unwavering support he showed our family through the hardest times. The world knew Robin as a comedic titan, but to our family, he was simply one of our Dad’s dearest friends. From the moment they were classmates at Juilliard, their friendship transformed into a brotherhood that was built on a mutual admiration for the theater, the arts and, most importantly, laughter.
"After our father's accident, Robin's visit to his hospital room was the first time that Dad truly laughed. Dad later said, 'My old friend had helped me know that somehow I was going to be okay.' Robin and his incredible family stayed by our side for the rest of Dad's life, and long after that as well. He and Marsha Garces joined the Board of the Reeve Foundation, and provided unending support without fanfare or question.
"Robin's unparalleled legacy on screen will last through the ages. But beyond the gift of laughter, he gave our family and the Reeve Foundation the gift of his simple, steadfast friendship. It's a gift we'll treasure forever."
For more on Robin Williams's tragic death and his legacy of comic genius, pick up the latest issue of PEOPLE, on newsstands Friday
If you or someone you know is considering suicide, please contact the National Suicide Prevention Lifeline at 1-800-273-TALK (8255).
The Williams family is asking well-wishers to send contributions to charities close to the actor's heart in lieu of flowers. Suggested organizations include St. Jude Children's Research Hospital, Challenged Athletes, USO, the Muhammad Ali Parkinson Center, the Christopher and Dana Reeve Foundation and Glide Memorial Church in San Francisco.