A Handler was most likely a special type of human created by the Tleilaxu to care for and breed Futars, in their war against the Honored Matres out in Human Scattering-space. For the futars who found themselves encaptured and imprisoned on the no-ship kept on Chapterhouse, they constantly clamored in their need to find "their Handlers".
Handlers were taller than most humans, had a keen sense of smell; and had darkened pigmentation around the eyes, akin to raccoons.
When the same no-ship, the Ithaca, found itself above an arboreal and lush world after a prescience-guided jump by the ghola of Miles Teg, the futars aboard the vessel began acting nervously, and Hrrm, their leader declared they had come to 'home', the home planet of the Handlers and Futars.
The leader of the Handlers on that particular part of the planet was named Orak Tho, and he invited the landing party of the Ithaca to observe a hunt of the Honored Matres, and invited the Ithaca futars to participate in the hunt. After the futars killed some Honored Matres, the Matres ended up killing one of the Handlers, who in actuality turned out to be and Enhanced Face Dancer, under the control of the Thinking Machine leaders Omnius and Erasmus.
It is not know if all Handlers were Enhanced Face Dancers, or if some Enhanced Face Dancers had been strategically placed by the Machines, on the Handler home world.