In "Stranger Things", what is the full name of Max Mayfield's older stepbrother?

Max Mayfield's older stepbrother in "Stranger Things" is Billy Hargrove. Billy moves with Max and his mother to Hawkins after Max's mother marries Neil Hargrove, making Billy Max's stepbrother rather than her biological brother.

Billy is introduced in season 2 as a volatile and intimidating teenager. He has a difficult relationship with Max and frequently bullies or threatens her, while his own aggression is linked to abuse from his father. Despite his cruelty, the series later reveals fragments of Billy's painful childhood and his memories of his mother.

In season 3, the Mind Flayer possesses Billy and uses him to recruit other victims and build a physical form. Max and the others try to reach him through his memories. At Starcourt Mall, Billy breaks free long enough to protect Eleven from the Mind Flayer, apologizes to Max, and dies from his injuries. Neil Hargrove is Max's stepfather, not her brother; Jonathan Byers and Steve Harrington are unrelated characters.