(coexisting on another dimensional plane)
These three unnamed Humans performed daily operations on the bridge of the Enterprise, including duties at the engineering station.
These two guards, along with Roger Lemli, stood guard on bridge of the Enterprise.
Chekov's guards attempted to kill Captain Kirk on Chekov's orders in the mirror universe, in order that the entire crew would advance in rank. They were both killed by Wilson, Chekov's third guard, who took sides with Kirk for an opportunity at a better promotion. (TOS: "Mirror, Mirror")
Several unnamed Humans were on duty patrolling, standing guard, or simply passing through in the corridors of the Enterprise between the transporter room and sickbay. Each would salute Captain James T. Kirk as he, Scott, Uhura and Doctor McCoy passed by. (TOS: "Mirror, Mirror")
This unnamed Human stood guard outside of the emergency manual monitor. He was incapacitated by Doctor Leonard McCoy's hypo, so that he and Montgomery Scott could enter the emergency manual monitor to manipulate the Enterprise's systems. According to McCoy, the dosage he gave the guard would have him put out for six hours. (TOS: "Mirror, Mirror")
Led by Farrell, Kirk's guards came to his rescue, however were already beat to the scene by the traitorous Wilson during Pavel Chekov assassination attempt. They later oversaw Chekov's punishment in the agony booth.
Phaser Control guard
This unnamed Human guarded the phaser control room. He confronted Montgomery Scott for not having authorization when Scott attempted to enter the room to disable the Enterprise's phasers. When the guard suggested contacting security chief Hikaru Sulu, Scott backed down to avoid a confrontation. (TOS: "Mirror, Mirror")
This unnamed Vulcan guard escorted Spock as he, James T. Kirk and Farrell walked the corridor. Spock later warned Hikaru Sulu that if he were to be assassinated that Sulu remember that Spock's operatives would avenge his death and that some of them were Vulcans. (TOS: "Mirror, Mirror")
This attempt was thwarted when Marlena Moreau used the Tantalus field against the guards, vaporizing them, allowing the four members of the USS Enterprise to return back to their universe. (TOS: "Mirror, Mirror")
Transporter room guards
The second guard was later on the bridge when Uhura engaged Sulu. Uhuru had him take over at her communications station after the altercation so that she could go to the sickbay. (TOS: "Mirror, Mirror")