Seekers are people who have been trained to kill the Soul Stealer. They have been brought up to utilize many tools and methods in order to capture the Soul Stealer, as well as kill it. They have also been brought up to recognize the Soul Stealer's particular Resonance Soul from any other. If the Soul Stealer uses his powers, the Seekers will know from miles away and go in that direction.
While there are many people who unofficially call themselves Seekers, the true Seekers have been contracted by The League.