(covers information from several alternate timelines)
Lieutenant Commander Yor was a 24th century Betelgeusian Federation Starfleet officer from an alternate reality created by a Romulan mining ship. He also was a time soldier who, in 2379, not only traveled forward in time, but across universes, arriving in the "prime" universe. Before the Terran emperor Philippa Georgiou time traveled to 3189 from 2258, Yor was the only individual known by the prime universe Federation to have traveled through both time and dimensions.
After his arrival he began to experience a severe medical condition caused by traveling across both time and dimensions, with his molecules attempting to return to either their time, or place of origin. Not only were his painful symptoms incurable, but returning Yor would have broken the "ironclad" Interdimensional Displacement Restriction, so his Doctors petitioned the Federation for euthanasia.
Despite this, Kovich described his suffering as "a breeze" in comparison to what Georgiou would face.