Which character is Joyce Byers's ex-husband?

Joyce Byers’s ex-husband is Lonnie Byers.

Lonnie is the biological father of Jonathan and Will Byers and Joyce’s former spouse. The marriage had ended before the main events of Stranger Things, and Lonnie is largely absent from the children’s daily lives when the series begins. His strained relationship with Joyce and the boys helps explain the family’s financial and emotional difficulties.

Lonnie appears in the first season when Will disappears. Rather than offering steady support, he disputes Joyce’s belief that something supernatural has happened, adding to the pressure she faces while searching for her son. His behavior reinforces the contrast between his detachment and Joyce’s fierce determination.

Viewers sometimes confuse Lonnie with Bob Newby, Joyce’s kind-hearted boyfriend in Season 2, or Jim Hopper, her eventual romantic partner. Murray Bauman is Joyce’s ally and investigator, while Ted Wheeler is Nancy, Mike, and Holly’s father. Lonnie’s defining connection is to the Byers family: he is Joyce’s divorced former husband and Jonathan and Will’s father.