Beckett Mariner was a hypothetical mirror universe version of Ensign Beckett Mariner in Mirror Universe Encounter, one of the training drill holoprograms presented by Shari yn Yem to the crew of the USS Cerritos in 2381.
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The prime universe Mariner in the guise of her Terran counterpart
In Mariner's run-through of the program, the premise was that she had taken the place of her Terran counterpart, and had to infiltrate the ISS Cerritos without arousing suspicion. Mariner failed the drill after she gave a Terran salute to Brad Boimler with her left hand, causing Boimler to expose her as an imposter, as the Terran Mariner was supposedly right handed. Unbeknownst to Mariner, Yem had rigged all the drills to ensure that the Cerritos crew would fail, suggesting there was no way for Mariner to learn this without being discovered. (LD: "I, Excretus")
This individual was only mentioned in dialogue.