Hama developed bloodbending after Fire Nation imprisonment.
Hama was a waterbender from the Southern Water Tribe who was captured during the Hundred Year War. In prison, Fire Nation guards deprived the waterbenders of ordinary water and restrained them to prevent escape. Hama eventually realized that living bodies contain water, allowing her to manipulate the fluids inside rats and, later, human beings.
She used the technique to control a guard and escape after decades of captivity. Hama’s story appears in “The Puppetmaster,” the eighth episode of Book Three: Fire. She later kidnapped Fire Nation villagers during full moons as revenge, forcing Katara to use bloodbending against her.
A common mix-up is Yakone, who was not the technique’s originator. He was a much later bloodbender in The Legend of Korra, where bloodbending had become a rare inherited ability. Hama is the canonical inventor shown in the franchise.