Humanoids with three sexes whose cogenitors, needed for reproduction, are denied rights, a fact that led to tragedy aboard Enterprise.
The Vissians are a humanoid species with three sexes. Along with males and females, a small number of cogenitors are needed for reproduction, but cogenitors are not taught to read and are treated as having no rights of their own in Vissian society.
In Cogenitor, Enterprise met a Vissian ship studying a hypergiant star. Trip Tucker befriended the cogenitor aboard, taught it to read and gave it the name Charles. When the cogenitor asked for asylum, Archer refused and returned it to the Vissians, and it later took its own life.