The Chenjesu are a race of silicon-based, crystalline beings native to the Sapphire World of Procyon 2. They are photo/chemovores, and have no natural predators, nor need for prey. They are naturally non-hostile.
They also have natural receptors in the HyperWave band, which gave them the first inkling of the Ur-Quan invasion. They formed a military alliance with the Mmrnmhrm to form the basis of the Alliance of Free Stars.
The Chenjesu were the first race to officially contact Humanity. Their star charts of HyperSpace form the standard that the races in this region of space use.
The Chenjesu, while silicon-based, are not at all like the Taalo -- Chenjesu are crystalline rather than rocklike, and unlike the entirely pacifist Taalo, the Chenjesu have no qualms about unleashing awesome destructive powers against those who threaten them. Their starship, the Broodhome, possessed more raw power (and raw material) than any other Alliance ship -- even if, in general, it was not the most effective in combat.
Images of the Chenjesu captains imply that they have limited control over electrical discharges, which they use to control their equipment.
After the Sa-Matra was deployed against them by the Ur-Quan, they and the Mmrnmhrm requested to be shielded together on the Chenjesu homeworld. The Ur-Quan permitted this, and the two races began to synthesize into the Chmmr.
The reasons for this are fairly clear for the Mmrnmhrm, as that race had lost the ability to reproduce on its own. The motivations of the Chenjesu are still unclear. The Role-playing Resource Guide indicates that the Chenjesu probably decided to do this because they felt their culture had become advanced enough to stagnate entirely, and that the modification of their species into a transChenjesu synthesis would reinvigorate their society.
The Chenjesu got along very well with the other races in the Alliance. In fact, they were the leaders of the Alliance, though they refused to take that title officially.