Blake Lively has penned a stirring tribute to her Gossip Girl costar Michelle Trachtenberg, who died today at the age of 39.

“This is the first day I met Michelle. She was electricity,” she wrote on her Instagram story over a still of the two conversing in character on the popular teen drama. “You knew when she entered a room because the vibration changed. Everything she did, she did 200%.”

Lively remembered Trachtenberg as someone who “laughed the fullest at someone’s joke, she faced authority head on when she felt something was wrong, she cared deeply about her work, she was proud to be a part of this community and industry as painful as it could be sometimes, she was fiercely loyal to her friends and brave for those she loved, she was big and bold and distinctly herself.”

Trachtenberg was discovered this morning “unconscious and unresponsive” in her apartment by authorities, who later pronounced her dead on the scene, the New York Police Department told Entertainment Weekly. Her representative Gary Mantoosh shared in a statement with EW, “It is with great sadness to confirm that Michelle Trachtenberg has passed away. The family requests privacy for their loss. There are no further details at this time.”

Trachtenberg and Lively appeared together on all six seasons of Gossip Girl, playing Upper East Side frienemies that just can’t quit each other. Their friendship went through many ups and downs, considering Trachtenberg’s character, Georgina Sparks, was one of the series’ most diabolical troublemakers. But Lively remembers Trachtenberg with only glowing positives.

“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,” she wrote.

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Several of Trachtenberg’s other Gossip Girl costars have shared remembrances in the wake of her passing.

“Michelle was one of a kind,” Chace Crawford wrote on Instagram. “I remember her coming on set for the first time and just absolutely owning it. She was a force of nature and just so so unapologetically funny and magnetic.. remembering those years with a big smile. Just a terrible loss. Love you.”

Ed Westwick posted a similar tribute to his own Instagram: “So sad to hear of the passing of [Michelle Trachtenberg]. Sending prayers.”