Mister Otrin was a scientist from an unidentified planet located in the Delta Quadrant that was devastated by the Earth probe Friendship 1. When the probe crashed on his planet, it released antimatter that poisoned the air and caused a nuclear winter.
When the planet was later visited by an away team from the Delta Flyer II, he attacked Harry Kim but was stunned by Chakotay and taken back to the USS Voyager. Once in Voyager's Sickbay, the Doctor determined that his tissues were saturated with antimatter radiation, therefore making him indistinguishable from the environment on his homeworld.
When presented by Kathryn Janeway the request of having her hostage-held crew returned from the surface in exchange for a cure, he accepted, and eventually took over the leadership of his people when their previous leader, Verin, refused to listen to reason. Voyager was able to reverse the effects of the radiation. (VOY: "Friendship One")