Kaj was a slave of the Tarasin species who possessed the Force-sensitive trait.
Around 20 BBY, Kaj, while still a child, was taken prisoner with three other Tarasin children, specifically Tiscak, Honna, and Li'Iri, because they all had the natural ability to use the Force. Phylus Mon, a Chevin who dealt in slavery, was their captor, and he held them captive in a cell within his slave zoo located on the ship called the Animiasma. Furthermore, he would whip each of the children using a whip that had three barbed tails.
The fact that they had vanished did not go unnoticed by everyone. Lanius Qel-Bertuk, a Jedi Master, had knowledge of the abductions, and he even had suspicions about Phylus Mon's involvement, yet he was powerless to save the children.
The children were confined to the cell, being the only Force-sensitives present in the zoo. They felt an immediate aversion to Halin Bok, another prisoner. Honna also found a way to communicate with the Mantellian Savrips in a neighboring cell, specifically Clu, Taf and Mev. The Savrips made a promise that they would break through the energy bars in order to rescue the children if anyone tried to take them against their will. Honna was fearful of such an attempt, as the energy bars could easily cause the death of the Savrips if they were to try to break through them.
Phylus Mon performed surgery to implant an explosive device near the heart of each of his slaves. If Phylus Mon were to die, the explosive would be triggered, but it could also be deactivated by the impact of an ion weapon. Additionally, all of the slaves held in Phylus Mon's zoo were routinely subjected to physical abuse.