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Sarah Michelle Gellar mourns Michelle Trachtenberg, her little sister on 'Buffy the Vampire Slayer'

The pair starred in the beloved series in the early 2000s. Trachtenberg played Dawn Summers, the sister of Gellar's title character.
The 25th Paley TV Festival - "Buffy the Vampire Slayer" - Arrivals
Sarah Michelle Gellar and Michelle Trachtenberg at a 2008 "Buffy the Vampire Slayer" reunion in Hollywood, Calif. Jean-Paul Aussenard / WireImage / Getty Images file

Actor Sarah Michelle Gellar paid tribute to her “Buffy the Vampire Slayer” co-star Michelle Trachtenberg on Thursday, a day after police found Trachtenberg dead in her New York City home.

Gellar posted a series of old photos on Instagram, showing the pair on the set and behind the scenes of the hit series.

“Michelle, listen to me. Listen. I love you. I will always love you," Gellar wrote. "The hardest thing in this world, is to live in it. I will be brave. I will live… for you."

Trachtenberg, 39, played Dawn Summers in “Buffy,” the younger sister of the title character played by Gellar, appearing in 66 episodes of the hit series from 2000 to 2003.

Gellar's post appeared to refer to a quote from the show: "Dawn, listen to me. Listen. I love you. I will always love you. But this is the work that I have to do. Tell Giles ... tell Giles I figured it out. And, and I’m OK. And give my love to my friends. You have to take care of them now. You have to take care of each other. You have to be strong. Dawn, the hardest thing in this world ... is to live in it. Be brave. Live. For me.”

The caption also included broken heart emojis, followed by a key emoji — most likely a reference to Trachtenberg's character being known as "the key" who can unlock multiple worlds.

Gellar is reportedly in talks to appear in a reboot of "Buffy the Vampire Slayer." It is unclear whether Trachtenberg would have returned to the series.

Trachtenberg, a beloved millennial actor also known for her work on the hit teen drama “Gossip Girl,” was found unresponsive in her home Wednesday morning, according to police.

She was pronounced dead at the scene, and no foul play is suspected, police said. The New York medical examiner's office said in a statement Thursday that the cause of death remains undetermined.

Trachtenberg's death sparked an outpouring of grief from Hollywood.

Blake Lively, who starred with Trachtenberg in "Gossip Girl" from 2007 to 2012, posted a tribute on her Instagram story Wednesday.

“She was electricity. You knew when she entered a room because the vibration changed. Everything she did, she did 200%,” Lively wrote. “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, 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.”