Teirna was a member of the Kazon-Nistrim and one of Seska's aides. USS Voyager answered a distress call from his shuttle in late 2372. His parents were killed in a Trabe attack shortly after his birth.
Chakotay's suspicions turned out to be correct, as Teirna was in reality a suicide bomber; what The Doctor believed to be polycythemia was in fact a process that turned his tissues into a high explosive. During the battle with the Kazon fleet, he used a needle embedded inside an artificial toenail to trigger the explosives, rendering Voyager helpless and allowing the Kazon to capture the ship. (VOY: "Basics, Part I")