While Netflix has stayed mum on the rumors, according to TV Line, "all signs point" to Brown once again being a series regular.
Brown's involvement in the second season was called into question in August after executive producers Matt and Ross Duffer confirmed the casting of Gaten Matarazzo (Dustin), Finn Wolfhard (Mike Wheeler) and Winona Ryder (Joyce Myers) for season two, but said Eleven's return was "up in the air."
"We don't know about Eleven," they told Variety when asked about the future of Brown's character, who disappeared with the Demogorgon at the end of last season. "We leave that up in the air."
Brown's addition to season two is the latest casting announcement for the series. Sadie Sink has joined the cast as Max, a "tough and confident" young female, while Dacre Montgomery will play Billy, her older brother. Joe Keery (Steve) and Noah Schnapp (Will) have also been promoted to series regulars.
Arriving in 2017, season two will pick up about a year after the events of season one, in the fall of 1984, and will focus more on the "bigger mythology" behind Will Byers' disappearance, including more of the Upside Down.
ET caught up with Brown, along with her Stranger Things co-stars Matarazzo and Caleb McLaughlin (Lucas) at the Emmys last month, where they dished on what they know about the show's sophomore season.