Alyson Hannigan says Michelle Trachtenberg "brought a loving energy" to 'Buffy The Vampire Slayer'.
The 50-year-old actress - who played Willow Rosenberg on the iconic teen drama - has paid tribute to her late co-star, who died this week aged 39.
Alyson wrote on Instagram: “I am deeply saddened by the news of Michelle’s passing.
“She brought a loving energy to the set of Buffy. My thoughts are with Michelle’s family and friends.”
Trachtenberg joined the series for its fifth season as Sarah Michelle Gellar's titular character's sister Dawn Summers, and she remained on the show until its conclusion with season seven in 2003.
David Boreanaz, who played vampire Angel, wrote on his Instagram Story: “So very sad …horrible news. RIP and prayers to her and her family.”
Emma Caulfield, who played former demon Anya Jenkins on the show, said the "Buffy family just lost a little sister".
She added: "I’m so sorry your bright light died so young.
"Our Buffy family just lost a little sister today… Rest in peace lovely Mish Mish. You were loved."
Kristine Sutherland, who starred on the show as Buffy and Dawn's mother Joyce, said in a statement: "I am devastated by the news of Michelle's death.
"The day she joined the Buffy family she brought a joy and innocence with her along with her talent.
"She was lovely to work with and will be missed. My heart goes out to her family."
Clare Kramer, who played villain Glory on the show, wrote: "Peace out little one. A talented kind soul gone way too soon! You’re free to fly now…"
Meanwhile, 20th Television - which is the studio behind the upcoming 'Buffy' reboot - shared in a statement to PEOPLE magazine: "Michelle touched a generation of television fans throughout her career, including in her unforgettable role as the independent and strong-willed Dawn Summers in 'Buffy'.
"She brought depth, heart, and authenticity to every performance and will be remembered for her remarkable talent.
"Our sincere condolences go out to her family and friends – she will be deeply missed."
Speaking to the same outlet, James Marsters (Spike in 'Buffy') said his "heart is heavy" and "we have lost a beautiful soul".
He added: "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.
"My heart goes out to her family who are good people, and are suffering the greatest loss anyone could bear."
Trachtenberg - who was also best known for starring on 'Gossip Girl' - was found dead inside her Manhattan apartment on Wednesday.
The New York Police Department (NYPD) has confirmed to PEOPLE that it responded to a 911 call at 1 Columbus Place in New York City.
The NYPD added: "Upon arrival, officers observed a 39-year-old female unconscious and unresponsive.
"EMS responded to the location and pronounced the victim deceased. Criminality is not suspected."
