Besides the original cast of the first season, it's difficult to argue that anyone got as much breakout stardom from Stranger Things as Season 4's Eddie Munson, played by Joe Quinn. The high school flunkie who ran a D&D group for children approaching half his age managed to tug at the heart strings of many fans, and his death has been the centre of some conspiracies.
Despite fan theories almost certain he'll return, Stranger Things creators The Duffer Brothers have confirmed via Empire he's not coming back. "I love that Joe Quinn is toying with people! But no, he's dead," Matt Duffer said. "Joe is so busy anyway, that everyone should know he's not coming back. He's shot like five movies since! When the hell has he got time to come and shoot Stranger Things? No, sadly, RIP. He's fully under that ground."
Quinn's death came towards the end of Season 4, where he was torn apart by bat-like creatures from the Upside Down. I'm no expert, but it's hard to see someone coming back from that. There were theories that would have seen Eddie return as an undead servant of Vecna, who would then turn on him at the end, but even if that had been in original story drafts, Quinn likely just has too much on his schedule to help people live out fanfictions.