The situating noun אִישׁ is the usual term for labeling a key participant (esp. an adversary) when framing a conflict situation. This is a special case of this noun’s prototypical use to label a participant as being essential for grasping the depicted situation. The use of this noun also evokes the conflict situation, which is more difficult to translate into English.
The discourse function of אִישׁ suffices to explain its presence: it prepares the audience for the referent’s further description that follows. Although in 2022 I added a footnote to provide an alternative rendering of this term, upon reflection I was attributing more informational meaning to it than is warranted. (But deleting the note at this point is more trouble than it’s worth.)