Humans are always evolving. The ones who evolved in the right path would embody superpower, and called the "White." However, the when their psyche can't keep up with the evolution of their body, they'll lose their mind and go berserk. They are called the "Black."
Jin is a part of Black Line; the security company against the Black.
Jin swears to eliminate all the "Black" from this world–– wether by finding a cure to end the spread of the "Black," or with his own fist and power.

(Source: MangaHelpers)