Sarah Michelle Gellar posted a tribute to her late Buffy the Vampire Slayer co-star, Michelle Trachtenberg, who passed away suddenly on Wednesday. The 39-year-old, who also acted in the TV-hit Gossip Girl, was found by her mother “unconscious and unresponsive” in her New York City apartment. Reports said that Trachtenberg recently underwent a liver transplant, and police were not investigating her death as suspicious. “Michelle, listen to me. Listen. I love you. I will always love you,” Gellar wrote in an Instagram post filled with photos of the pair together. “The hardest thing in this world, is to live in it. I will be brave. I will live… for you." She finished off the post with broken hearts and a key emoji. The quote is from a popular scene in the Buffy series in which Gellar’s character dies, saving Trachtenberg. Gellar is not the only co-star to pay tribute to Trachtenberg. Gossip Girl alum Blake Lively wrote that she was “blindsided” by the death. “The world lost a deeply sensitive and good person in Michelle,” Lively said on Instagram. “May her work and huge heart be remembered by those who were lucky enough to experience her fire.”