Final answer:
In 'The Emperor Jones' by Eugene O'Neill, Jeff is the imaginary minor character who appears in the protagonist's hallucinations, providing insight into the protagonist's past.
Step-by-step explanation:
The character considered an imaginary minor character in the play 'The Emperor Jones' by Eugene O'Neill is Jeff. Henry Smithers is a white trader and secondary character in the play who interacts with Brutus Jones, the protagonist, while Lem is the leader of the rebellion against Jones. However, Jeff is a character from Jones's past, who appears only in his hallucinations and memories throughout the play. As such, Jeff does not physically interact with other characters on stage and is considered part of Jones's internal struggles and recollections throughout the narrative. The imaginary minor characters like Jeff play a significant role in revealing the protagonist's emotions and backstory.