The Duffer Brothers say that surprise character return in Stranger Things "immediately changes the tone" in season 5 Vol. 2: "I think it'll be surprising for people to see how she ultimately fits into the story"

That surprise return of Eleven's 'sister' in Stranger Things season 5 Volume 1 had some fans (me) in tears – and the Duffer brothers say this is only just the beginning of her

"It will be a very different dynamic going into the second volume," Matt explains in the latest issue of SFX magazine. "Kali shifts things a lot for Eleven, and I think it'll be surprising for people to see how she ultimately fits into the story. We're excited about that, because it immediately changes the tone. And obviously, the way things ended with Hopper leaves a lot of tension between them."

Kali was first introduced in what is known as a "bottle" episode, where an episode of a television show with an ensemble cast focuses only on the perspective of one particular cast member. In this case, Eleven went off on her own, met up with Kali and her crew, and learned that there are other people out there just like her who are living normal lives. Kali was also experimented on at Hawkins Lab, and has the number eight tattooed on her wrist. She's just as powerful, if not more powerful, than Eleven.

"Bringing back Kali was something we had always been talking about for a long time, but we had just never quite figured out how to do it," he continues. "We were working on season five, and we were talking about making sure every storyline was finished. It was always nagging at us that the Kali storyline was hanging. So it was important to revisit it and complete her arc."

Director Shawn Levy acknowledged that although the episode received some pretty mixed reviews, it was important to bring Kali back – and fans agree. After Eleven failed to defeat Vecna at the end of season 4, several theories popped up that suggested Eleven would need Kali's help in order to do so.

"It was a risky bet to try a standalone episode with largely new characters in the middle of our second season. It worked for some viewers," Levy said. "It didn't work for others, but I respect the hell out of the Duffers for taking that big swing. But we never forgot about Kali. We never forgot about the plot strands that got established."

Stranger Things season 5 volume 1 is out on Netflix, with volume 2 set to arrive on Christmas Day. See our Stranger Things season release schedule for exact timings. For more, check out our Stranger Things season 5 volume 1 review, and the best Netflix shows to stream right now.