Sarah Michelle Gellar Says She Will ‘Live for’ Michelle Trachtenberg in Touching Tribute toBuffyCostar: ‘I Will Always Love You’

Trachtenberg, 39, was found dead in her apartment on Feb. 26

Sarah Michelle Gellar/Instagram

is mourning the loss of.

On Thursday, Feb. 27, the day after, Gellar, 47, paid tribute to her with a carousel offrom their time onBuffy the Vampire Slayertogether.

“Michelle, listen to me. Listen. I love you. I will always love you,” Gellar wrote of Trachtenberg, who was 39, quoting her character’s last line from the series. “The hardest thing in this world, is to live in it. I will be brave. I will live… for you.” She added “💔💔💔🗝️” to the end of the caption.

Trachtenberg joinedBuffyas Dawn Summers, the kid sister to Gellar’s Buffy Summers, in season 5. Her existence in the show was explained by her being a mystical “key” put into a human body and sent to the Slayer for protection against the evil god Glory (Clare Kramer).

She stayed on the show through its finale in 2003, but Gellar sacrificed herself in the season 5 finale — and told her sister, “Dawn, the hardest thing in the world is to live in it. Be brave. Live — for me.”

Sarah Michelle Gellar/Instagram

Trachtenberg was found “unconscious and unresponsive” in her apartment around 8:00 a.m. on Feb. 26, the NYPD confirmed to PEOPLE. She was pronounced dead at the scene and an investigation remains ongoing, but criminality is not suspected, per police.

A source told PEOPLE thatover the last year.

“She told friends she was struggling. She was really, really sick and open with those in her circle about how much she was struggling,” the source said.

Sarah Michelle Gellar/Instagram

In the aftermath of her death,.

was among the first cast members to react to the news online, writing on his Instagram Stories, “So very sad …horrible news. RIP and prayers to her and her family.”

, who played the vampire Spike on the show and shared many scenes with Trachtenberg, said in a statement shared with PEOPLE: “My heart is heavy today. 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,” he continued. “My heart goes out to her family, who are good people and are suffering the greatest loss anyone could bear. I hope everyone can give them space to heal in this most difficult time. Godspeed Michelle. You are missed.”

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, who starred as Willow Rosenberg on Buffy, also shared a touching tribute on, writing, “I am deeply saddened by the news of Michelle’s passing. She brought a loving energy to the set ofBuffy. My thoughts are with Michelle’s family and friends.”

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