Penn Badgley has recalled how the late Michelle Trachtenberg always had a "pure heart".
The Buffy the Vampire Slayer actress passed away in February at the age of 39, several months after she underwent a liver transplant.
Authorities reported Trachtenberg died from complications of diabetes, which can be a side effect of organ transplantation.
Reacting to the sudden loss of his Gossip Girl co-star at the season premiere of Netflix's You on Wednesday, Penn remembered her as always being "really joyous".
"It's surreal. What I remember of her is she was incredibly quick to laugh," the actor, who portrayed Dan Humphrey on the show, told E! News at the event. "She was really joyous. We all have our ups and downs, our lights and shadows, but she had this - I mean, she was mature - but she had this childlike, joyous laugh."
Penn went on to admit that he hadn't stayed in touch with Trachtenberg, who played Georgina Sparks, after they wrapped filming Gossip Girl in 2012.
However, the star insisted he would always remember his former colleague's "really pure heart".
"I can't claim to have known her well, but yeah, Really surreal, really saddening," the 38-year-old added.
The fifth and final season of You begins streaming via Netflix on Thursday.