I am writing a scene where a character interacts with someone he cannot see and does not know - in essence anonymous stranger, with no visual aids. Are there any other words or phrases I can use to refer to the second party instead of "the other"? Because of the lack of visuals, I cannot say "the man with the hat" or something similar, and since positions relative to each other are important, I cannot accurately depict their motions/actions relative to each other in detail without using "the other." A lot of motions are going on and I'm sick of using the same repetitive phrase.
For example, "The other flipped him over their shoulder."
Either a phrase or a single word replacement works fine, I just want to have something that conveys the meaning without giving away detail that the character doesn't have that isn't "the other."