(coexisting on another dimensional plane)
A cunning schemer with designs on overthrowing Kirk as captain, he enlisted several other crew members in his mutiny. By the time that he carried it out, however, Kirk had been replaced by his counterpart from a parallel universe who was able to defeat him. He was punished with a trip to the agony booth. Although his actions warranted an execution, the Kirk of the primary universe spared his life, ordering him to be released from the agony booth and confined to quarters. (TOS: "Mirror, Mirror")
Background information Edit
In William Shatner's novel Dark Victory, the mirror Kirk - by this point known as "Emperor Tiberius" - mentions to his prime universe counterpart that when he returned from the crossover, he found Chekov still in the agony booth, his torturers at a loss because of the restraint the other Kirk had shown. Tiberius states that he set the agony booth to medium intensity - "like a dull toothache throughout your whole body. Bearable, but most uncomfortable" - and left Chekov in it; it would take thirteen days for him to die, and it set a new standard for the Imperial Starfleet.
Chekov is one of only two of the original series regulars' mirror counterparts who does not appear in the Star Trek: Mirror Universe novel The Sorrows of Empire (the other is Sulu, who is killed by Spock's loyalists "off-screen"), and the only one not mentioned by anyone in the narrative.
Another alternate vision of the mirror universe appears in the video game Star Trek: Shattered Universe, which shows Chekov surviving and rising to the rank of commander, serving as captain of the ISS Enterprise-A as it pursues the ISS Excelsior across the galaxy.