In Stranger Things Season 3, Billy Hargrove, played by Dacre Montgomery, plays a significant role in the storyline.
Towards the end of the season, Billy becomes the host for the Mind Flayer, the main antagonist in the series. The Mind Flayer uses Billy to do its bidding and spread chaos in Hawkins.
In the final episodes, the kids, along with Eleven and her friends, confront the Mind Flayer at the Starcourt Mall. During the battle, Eleven uses her powers to reach the real Billy trapped within the Mind Flayer's control. She reminds him of a memory from his past, specifically a moment with his mother. This emotional connection helps Billy briefly break free from the Mind Flayer's influence.
Unfortunately, the struggle is short-lived, and Billy ultimately sacrifices himself to save Eleven and the others.
Complete question
In Stranger Things Season 3: What Happened to Billy?