(coexisting on another dimensional plane)
Because he was a slave, the Kelpien did not actually have a name. However, in the mid-2250s Michael Burnham of the prime universe decided she was going to call him Saru, in honor of a respected friend.
While Burnham communicated with the primary universe Saru, he asked if there were any Kelpiens on board the Shenzhou. Burnham decided not to tell Saru about the existence of his mirror counterpart.