Actress Michelle Trachtenberg, who has died aged 39, is most famous for two very different roles in two very different shows that both loom large in the millennial consciousness. She was a fifth season addition to Buffy the Vampire Slayer, taking on the role of Buffy’s mysterious sister Dawn, a complicated, maturing young woman who came to occupy a central role in the series’ mythology. Four years later, she began a recurring role on Gossip Girl, as the Cruella-squared pot-stirrer Georgina Sparks, whom Trachtenberg once described as “basically the evil bitch from hell”.

Michelle Trachtenberg: a life in pictures

With their very different aesthetics and intended audiences, Trachtenberg’s two hit series speak to very different parts of her fanbase: Buffy has a campy, B-movie charm, whereas Gossip Girl is as slick and indulgent as a delicious dessert. As those two shows indicate, her career defied easy generalisation – what to say of the woman who had her breakout role aged 11 in Harriet the Spy, before moving on to the racy teen raunchfest EuroTrip and the wholesome Disney banger Ice Princess?

Trachtenberg managed to rack up a dizzying string of major performances before she had even turned 20 years old – indeed, Gossip Girl’s Georgina was one of the few big roles that showed her past her teenage years. She started acting early, a natural who did not receive any formal training, beginning her career with a role in Law & Order when she was six. By all accounts she showed herself to be precocious and extremely hard-working.

Michelle Trachtenberg in Harriet the Spy. Photograph: NICKELODEON/Allstar

Although Trachtenberg was widely liked off-screen, she convincingly played mean-spirited characters. On Gossip Girl, Georgina excelled at delivering cutting remarks with a surgical precision that was truly pleasing to watch, all the while remaining so composed and self-possessed as to not displace a single hair.

And on Buffy, Trachtenberg’s Dawn was almost the anti-Georgina: a maturing girl struggling to find herself, occasionally a little bratty, but fundamentally showing a good and evolving soul. She was able to have the kind of deeply emotional, vulnerable, and reciprocal relationship with Buffy that would have been poison to a woman like Georgina.

Dawn worked so well precisely because she lacked the extreme self-possession that Georgina seemed to have in endless quantities – two characters as distant from each other as Gossip Girl’s icy socialite New York is from Buffy’s funky, supernatural California.

Leighton Meester and Michelle Trachtenberg in Gossip Girl. Photograph: Everett Collection Inc/Alamy

Trachtenberg fans tend to have very distinct preferences for either Dawn or Georgina, and probably for most people this is a very instinctual thing. Do you side with the young woman still figuring herself out, or the grown up mean girl who has long since decided she likes being a bitch? The answer is something of a personality test.

That Trachtenberg could contain these multitudes over the course of her career probably says as much about the fact that she essentially grew up on screen as it says about her capacious acting skills. She was the fortunate child star to make a successful pivot into an adult career, although in the years after Gossip Girl her career slowed, landing fewer roles as the millennial culture that she helped shape gave way to gen Z.

In her tribute on Wednesday, Trachtenberg’s Gossip Girl co-star Blake Lively feted her as someone who lived life with exuberance. “Everything she did, she did 200%. She laughed the fullest at someone’s joke, she faced authority head on when she felt something was wrong … she was fiercely loyal to her friends and brave for those she loved, she was big and bold and distinctly herself,” Lively wrote. It was a fitting tribute, from one millennial to another, to a woman who clearly understood what a privilege it was to act, who had relished her time in the spotlight.