William Shakespeare's Romeo and Juliet: Benvolio is an important character up until the end of act III. After that he's never seen or mentioned again. In some versions of the story he shows up at the very last scene but in most he just disappears. What happened to him? Did he decide to leave Verona? Or did Shakespeare just forget/ignore him?
Reynaldo, in Hamlet, appears in one scene being given detailed instructions to watch Laertes and report his habits and misdeeds. He leaves and is never heard from again, even after Laertes comes back.
In King Lear, The Fool is Lear's loyal companion when he is abandoned by everyone else, even braving the storm with him. He vanishes without a trace or mention after the third act. Some commentators regard the Fool as a functional replacement for Lear's loyal daughter Cordelia, who is banished in the first act and is reunited with her father in the fourth.
In Spring Awakening, Martha's fate is never mentioned after The Dark I Know Well.