An empathic metamorph was a humanoid who had an increased empathic ability and a natural desire to please others. This was often perceived as being the "perfect mate" since the individual naturally changed its persona and behavior to meet another being's emotional requirements and sexual desires.
Kriosian empathic metamorphs, through random genetic mutations, were relatively common, but the vast majority were male. Females appeared once in every seven generations. As a result, such females were highly-prized as mates; they were forcibly adopted by the state from age four and put through a rigorous educational program to further increase their value as mates. Kriosian metamorphs went through a complex three-stage sexual maturation process. The third stage was called the Finiis'ral. Soon after, the metamorph "bonded" with the person nearby, imprinting only their desires. Putting the metamorph in stasis could prolong the maturation process.
Garuth was a female Kriosian empathic metamorph who was loved by two brothers, Valt and Krios. Their fight over her started the Valtese-Kriosian war. Centuries later in 2368, another metamorph, Kamala, was given to Alrik to end it. (TNG: "The Perfect Mate")