He was one of the seven rogue operatives whom the Vulcan Ministry of Security had assigned T'Pol to capture. T'Pol was able to apprehend five of the operatives, but Jossen and another operative, Menos, managed to escape.
Several years later, T'Pol once again caught up with the operatives, this time tracking them to the planet Risa. Believing the operative was reaching for a weapon, T'Pol shot and killed Jossen. Menos, however, escaped once again.
Distraught over the fact that she had taken a life, T'Pol undertook the fullara to suppress her memories of killing Jossen. These memories resurfaced in 2152, when T'Pol apprehended the seventh and final operative, Menos. (ENT: "The Seventh")