Blake Lively is mourning the loss of her “fiercely loyal” and “brave” Gossip Girl co-star Michelle Trachtenberg, who has passed away at age 39. “This is the first day I met Michelle. She was electricity. You knew when she entered a room because the vibration changed,” Lively, 38, shared on Instagram on Wednesday, February 26, alongside a throwback photo from Gossip Girl. “Everything she did, she did 200%. She laughed the fullest at someone’s joke, she faced authority head-on when she felt something was wrong, she cared deeply about her work, and she was proud to be a part of this community and industry—despite its challenges. She was fiercely loyal to her friends and brave for those she loved. She was big and bold and distinctly herself.” Reflecting on her time with Michelle Trachtenberg, Blake Lively recalled how her co-star would “always” wear “yummy caramel-smelling lip gloss” because she didn’t just want to shine on camera—she wanted to create a warm experience for everyone around her. “She cared about the sweet details,” Lively shared. Calling Trachtenberg “kind, through and through,” Lively opened up about the pain of unexpected loss. “Time passes. You take for granted that you’ll get the chance to see an old friend,” she wrote. “To paraphrase, the real tragedies in life are the ones that blindside you on an idle Tuesday. Hold those you love and have loved dear. The world lost a deeply sensitive and good person in Michelle. May her work and her huge heart be remembered by those who were lucky enough to experience her fire.” Blake Lively played Serena van der Woodsen for all six seasons of Gossip Girl from 2007 to 2012, while Michelle Trachtenberg, known for her childhood roles in Ice Princess, Harriet the Spy, and Buffy the Vampire Slayer, joined the show as the scheming Georgina Sparks in Season 1 and remained a recurring presence until the series finale. Earlier on Wednesday, police sources confirmed to Us Weekly that Trachtenberg was found unconscious in her New York City apartment. No cause of death has been reported, but authorities do not believe the incident to be suspicious.