Kala was the wife of Aydan.
Kala, Aydan and Eyesen conspired to steal one of the keys to the Conscience of Marinus, hoping to sell it. After Aydan's part in the plot was revealed, she shot him so he would not reveal the names of his accomplices. She kidnapped Susan to cover up the crime.
Barbara Wright exposed her when Kala was confronted and already knew Susan was a hostage; as she could not have known that Susan was missing, Barbara deduced that Kala was somehow involved. When the First Doctor exposed Eyesen, he and Kala confessed everything. (TV: The Keys of Marinus)