Billy may be one of the more tragic characters on this list. Like Will, Billy is subject to a season-long cycle of torment from the Mind Flayer. Billy is first introduced to us in season two, when he and his stepsister, Max, move to Hawkins. Billy has a hard time integrating and, on the surface, he comes off as aggressive, angry, abusive, and all-around frightening. Billy doesn't get much character development until season three, when he has an unfortunate run-in with the Mind Flayer. Like Will acted as his agent in Hawkins in the previous season, that honour goes to Billy this time around.
Throughout the season, we see Billy corrupting the citizens of Hawkins to help the Mind Flayer build itself a body in our world, but we also see the tragic truth behind his character. His loving mother left him with his father when he was quite young, and his father grew incredibly abusive. His anger towards Max and her friends starts to make sense when you realise that he's emulating his own abuse, making him the perfect host for the Mind Flayer. Billy does get his redemption in the final episode of season three when he finally stands up to his tormentor to save his little sister and her friends from the monster he helped create.
Sadly, Billy doesn't survive the face-off with the Mind Flayer, but it isn't the last we see of him. Billy continues to be a pawn of the Upside Down, appearing in front of Max in flashbacks of the day he died, as well as through season four's antagonist, Vecna. Vecna takes the form of Billy while trying to add Max to his list of victims. It nearly works, as Max's guilt over Billy is hard for her to let go of. Billy may not be "alive," but his memory is enough for the overwhelming power that is the Upside Down.