Sarah Michelle Gellar mourned Michelle Trachtenberg on what would have been the actress' 40th birthday.

Gellar, who played the titular role on "Buffy the Vampire Slayer" in the '90s and early 2000s, reflected on decades of friendship with her onscreen little sister in an Oct. 11 Instagram post that included a clip of the two hugging on the WB show.

"When you turned 16, it was hard not to still see you as the little girl I met on All My Children. When you turned 21, I had to teach myself to not see you as the little sister anymore," Gellar, 48, wrote in the caption. "I can’t imagine what 40 would have been like. This I know … it would be filled with love. Happy birthday @michelletrachtenberg."

Trachtenberg died Feb. 26 at age 39. The New York City Office of Chief Medical Examiner released a statement in April announcing the cause of the "Gossip Girl" actress' death as complications from diabetes mellitus. The manner was determined to be natural.

Trachtenberg was found dead in a New York City luxury apartment building at 8 a.m. Feb. 26, a spokesperson for the New York Police Department previously confirmed to USA TODAY. Upon arriving at her unit, officers found her unconscious and unresponsive; EMS officials responded to the scene and found the actress dead.

Michelle Trachtenberg 'brought a loving energy' to 'Buffy the Vampire Slayer'

Just a few years after her film debut in 1996's "Harriet the Spy," Trachtenberg landed another one of her most memorable roles as Dawn Summers, the younger sister of Gellar's Buffy on "Buffy." She joined the show in the fifth season, despite Buffy never having had a sister before that, and stayed around through Season 7.

After news of Trachtenberg's death became public, "Buffy" co-star James Marsters – who played Spike – paid tribute to her in a heartfelt Facebook post.

"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," Marsters wrote.

Alyson Hannigan, who played Willow on "Buffy," also took to Instagram to remember her costar.

"I am deeply saddened by the news of Michelle's passing," wrote Hannigan alongside a series of photos of the pair. "She brought a loving energy to the set of Buffy. My thoughts are with Michelle's family and friends."

Contributing: Brendan Morrow and Jay Stahl

This article originally appeared on USA TODAY: Sarah Michelle Gellar remembers 'little sister' Michelle Trachtenberg on 40th birthday

Reporting by KiMi Robinson, USA TODAY / USA TODAY

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