James T. Kirk was a human male and denizen of the mirror universe. Introduced in the Star Trek episode Mirror, Mirror, Kirk was that universe's counterpart of James T. Kirk.
Like his prime counterpart, he was portrayed by William Shatner.
An officer in the Terran Empire's Imperial Starfleet, Kirk became first officer of the ISS Enterprise in the mid 2260s, under the command of Captain Christopher Pike. Kirk would later assassinate Pike and assume command of the Enterprise, with the mirror universe version of Spock acting as his first officer.
In 2267 the mirror universe versions of Kirk, Montgomery Scott, Nyota Uhura, and Leonard McCoy were all transposed with their prime universe counterparts when they beamed up from Halka during an ion storm. While their prime universe counterparts were more or less able to blend in, Kirk and his mirror universe compatriots were quickly exposed by the prime universe Spock, who had all four of them taken in to custody and placed in the brig.
Furious, Kirk threatened to have Spock and security guards executed for treason, telling Spock he'd hang the Vulcan by his ears. When Spock and the guards were unmoved by Kirk's threats, Kirk tried to bribe his way out of the brig, but found Spock equally unmoved by Kirk's promises of power and wealth.
Spock placed Kirk and the mirror universe crew in the transporter chamber, knowing that sooner or later the prime universe counterparts would activate the transporter and beam everyone back to their home universes.
After returning to the prime universe, the prime Kirk remarked that Kirk would find things changed if he had "read his Spocks correctly."
Kirk's fate following the transposition varied in various licensed literature.
Following his return home, Spock tried to convince Kirk to spare the Halkans, but was unsuccessful. Spock assassinated Kirk in his quarters and used the Tantalus Device in Kirk's quarters to dispose of Kirk's body. Spock assumed command of the Enterprise, and as a number of Kirk's other supporters "disappeared" he was able to consolidate his power and assume leadership of the Empire.
After the events of Mirror, Mirror Kirk continued in command of the Enterprise. Spock transferred off the ship a short time later and took on a leadership role at Starfleet Headquarters. Resentful of Spock's increasing power, Kirk fabricated charges of treason and had evidence planted at Spock's residence. Spock was tried and executed. With Spock's death the reform efforts died and the Empire continued its conquests for another century.
Spock decided against assassinating Kirk, knowing that his life expectancy would be measured in hours if he willed Kirk. Spock convinced Kirk to spare the Halkans on the grounds that the Halkans were a Klingon client state and it might lead to war with the Klingons if he destroyed them. Acting as the power behind the throne, Spock arranged for Kirk to become Emperor Tiberius I, then deposed him. Angered over this, Kirk went to the Klingons and Cardassians, convincing them to form an alliance to overthrow the Empire so he could resume his leadership.