Michelle Trachtenberg joined Buffy the Vampire Slayer so suddenly that it was a both a punchline and a cliffhanger.

At the end of the first episode of Buffy’s fifth season, a funny one called “Buffy vs. Dracula” where our heroine meets the world’s most famous vampire and her pal Xander gets “funny syphilis,” Buffy comes home to find what looks, to us, like a stranger rummaging through her room. When her mother calls out telling her to take her sister along on a date, both Buffy (heretofore an only child) and this unfamiliar young woman react with irritation – but not surprise. The cliffhanger: Buffy seems to already know this girl named Dawn, accepting her as her younger sibling. We, the audience for the preceding 78 episodes, do not.

For the next bunch of episodes, the show cleverly holds off on explaining where Dawn comes from, mostly treating her as a series regular. It plays a bit like a spoof of shows that desperately add a younger character to the cast later in their run, for additional youth appeal and/or cuteness – and many Buffy fans responded in kind, complaining about Dawn being a brat, some maintaining that position even after the truth about her is revealed. Eventually, Buffy and then Dawn find out that Dawn isn’t “real,” in the sense that she was not actually birthed by their mother or actually living alongside Buffy for fourteen years. She’s the Key, a magical entity given the form of a human, to assure that Buffy would protect it fiercely; for this to work, Buffy and everyone around her need to have false memories of Dawn’s presence, which of course Dawn herself shares, and then must question. Here, the show takes another hoary TV tradition – the retcon – and again both spoofs it and makes it poignant, with a character so unmoored by her wavering sense of reality that she commits self-harm at one point. Yet the gambit works, and Buffy is willing to sacrifice herself to protect her made-up sis.

But all the clever ideas in the world can’t be carried off without a capable cast, and this is where Buffy got lucky with its hiring of Michelle Trachtenberg, who died unexpectedly today, just shy of 40. Trachtenberg was a child star, working since the age of three, eventually finding some fame as a supporting player on The Adventures of Pete and Pete and as a beloved children’s book character in Harriet the Spy, both from Nickelodeon. Buffy, then, for which her co-star Sarah Michelle Gellar apparently suggested her, was a first real step toward adulthood. In contrast to the adorability one might expect from a practiced child performer, Trachtenberg played Dawn with a realness and rawness, befitting a show that took a cartoon premise (Cali blonde, like, fights vampires with her friends!) and grounded it in the emotional realities of high school turmoil – and, in those later seasons, the unsettling world beyond it.

So of course some fans thought Dawn was a brat, even after they learned the truth about her character; she was authentically bratty in a way that Trachtenberg made absolutely true to the character’s revised age of 14. (Initially, she was written closer to 12, but the character was adjusted for its actress, something that paid off throughout the rest of the show’s run in terms of Dawn’s, and Trachtenberg’s, increasing complexity.) A lesser show might have caved to fan reaction, or simply craved the status quo, and found a way to dismiss Dawn once the Key storyline was resolved at the end of the season. But Dawn stuck around for the rest of the series, getting her shot at a real life, growing and changing with the rest of the ensemble. Some of Trachtenberg’s best moments in the series come in the seventh season: In the episode “Him,” she gets to play broader comedy than usual as Dawn becomes despondently besotted with a jocky classmate, not realized she’s been enchanted by a magical jacket; in “Conversations with Dead People,” she performs a mostly-solo story where she thinks her dead mother is attempting to contact her from beyond. And perhaps Trachtenberg’s best episode comes with “Potential,” where Dawn allows herself to hope that she might be a “potential” Slayer – one of many young women who have latent vampire-fighting powers, who Buffy is gathering to fight a great evil. Her quiet, unshowy disappointment upon realizing that she’s “only” a normal girl is especially affecting given Dawn’s heart-on-sleeve teen-girl emoting of earlier seasons, and leads to a wonderfully written and performed scene between Dawn and Xander, the two non-supernatural normies in the Buffy gang, letting each other know that they’re seen.

That’s precisely what Trachtenberg brought to the series throughout her time on Buffy: a window into the normal teenagehood Buffy would never have, which comes with its own tumult and disappointment. As other characters grew into adulthood, Dawn remained, yes, full of potential, even if it wasn’t the vampire-fighting kind, and Trachtenberg, who started on the show at Dawn’s age of 14 and was nearly 18 by the time it was finished, always felt authentic in the role. Part of what’s so heartbreaking about losing her (besides, of course, the inherent sadness of someone dying young) is the reminder of just how human she was, no matter the Buffy fan’s picture of Dawn’s resilience. The character became as mortal and fragile as anyone else on the show, yet despite all the peril, she never died, or went completely dark, or turned undead. Dawn, more than any of the other characters, and despite her mystical origins, became the embodiment of the humanity Buffy was fighting for. Trachtenberg gave off that impression of strength in real life, too, having come out the other side of child stardom working steadily on the likes of Gossip Girl and Six Feet Under, and movies like Eurotrip and Mysterious Skin, where she played the best friend of fellow kid star Joseph Gordon-Levitt. That’s more than so many actors are able to manage: multiple memorable characters, including a role on all-time classic series. But it still doesn’t seem fair; she still had the potential for more.

Jesse Hassenger (@rockmarooned) is a writer living in Brooklyn. He’s a regular contributor to The A.V. Club, Polygon, and The Week, among others. He podcasts at www.sportsalcohol.com, too.