Welcome to Humor Nation. Billy Hargrove made his debut in the second season of Stranger Things, he was a secondary antagonist in the season. He had a troubled childhood as his father was physically abusive towards him. His mother left his father because of abusive behavior. This is why his dad married a woman named Susan who became his step-mom, she is the mother of Max Mayfield.
It was because of his father’s behavior, he turned into a bully. He arrives in Hawkins as his family moves from California. Speaking about his personality, he is suave and captivating, he has an unpredictable and violent nature and this is known to the people who are close to him. Billy also is antagonistic towards Steve and sees Harrington as a rival. He also holds ill feelings towards Lucas displaying signs of racism. He warns his step-sister Max to stay away from Sinclair.
When the Mind Flayer flays him, he becomes a part of it and gains some inhuman abilities such as enhanced strength and enhanced durability. His mind also became connected to the Mind Flayer’s mind. The finale of season 3 saw Billy Hargrove sacrificing himself in order to protect the life of Eleven and his friends including his stepsister Max. This gave Joyce enough time to seal the gate and ultimately killing the Spider Monster. While dying, with his last breath he apologizes to his sister Max.