Michelle Trachtenberg’s boyfriend, 63, speaks for first time after her death as haunting bullies video resurfaces

Friends and former co-stars of Trachtenberg's were quick to post tributes on social media honoring the actress.

David Boreanaz, who starred alongside Trachtenberg in Buffy the Vampire Slayer, posted a photo of the actress on his Instagram story, commenting, "So very sad...horrible news. RIP and prayers to her and her family."

Trachtenberg's Gossip Girl co-star Ed Westwick posted a photo on his Instagram story of the actress when she played Georgina Sparks. He wrote, "So sad to hear of the passing of Michelle Trachtenberg. Sending prayers."

Kim Cattrall, who starred alongside Trachtenberg in 2005 film Ice Princess, wrote, "Rest in peace sweet Michelle."

Rosie O'Donnell, who acted with Trachtenberg in Harriet the Spy, also said the news was "heartbreaking."

"I loved her very much. She struggled the last few years. I wish I could have helped," she said, according to People.

James Marsters, another co-star on Buffy the Vampire Slayer, posted on Instagram to honor the late actress. "My heart is heavy today," he wrote.

"We have lost a beautiful soul. Michelle was fiercely intelligent, howlingly funny, and a very talented person. She died much too young, and leaves behind scores of people who knew and loved her."

Fellow Nickelodeon star Kenan Thompson posted a farewell message to the actress and reminded others to check in on their friends.

"Our first Nick movie star has departed us!! She was my friend and now she rests!! Check on your people!!!" he wrote.

Melissa Joan Hart, Trachtenberg's Clarissa Explains it All co-star, said she was "heartbroken" over her death alongside a clip of the show.

"I'm heartbroken to hear of the passing of @michelletrachtenberg," she said. "So young, so talented and so sweet!"

"I think this was about 1992 and the episode was an idea I had about babysitting a total nightmare. But Michelle was nothing like her character and even this young, we got along wonderfully."