Yosa was a Maquis fighter who served on the raider the Val Jean under Chakotay until 2371 when his crew became stranded in the Delta Quadrant following the destruction of their ship. As a result the crew of the SS Val Jean, Yosa included, joined the crew of the USS Voyager as Starfleet personnel and traveled on the USS Voyager on its historic trek home from the Delta Quadrant. (VOY: "Caretaker")
In 2377, he was attacked by Tuvok, who was under the mental control of a Bajoran vedek named Teero Anaydis. Tuvok mind-melded with Yosa, and other Maquis, inciting them to mutiny and overthrow Captain Janeway. (VOY: "Repression")
Played by Mark Rafael Truitt, Crewman Yosa wore on his collar the provisional rank insignia quizzically labeled as "chief warrant officer" (a single blacked out stripe in a lozenge device) in the Star Trek Encyclopedia, but was referred to as "crewman," even though his insignia should have been equivalent to the hollow pip insignia worn by Miles O'Brien, which signified chief petty officer almost every time it was seen. This oddity persisted in VOY, every time a Maquis enlisted crewman was shown, creating the impression that all non-commissioned provisional personnel wore this insignia, despite what the Encyclopedia says about its use in reference to warrant ranks.