Sting

Sting is, with Prism, the starting antagonist, and they're capture will be the main objective of the first act of the game.

Trough the story
The first time you will "meet" them will be in the Savannah after defeating the boss in the mine of the Third Floor. After that you will be charge to find them, thing you will finally do in the Floor 8 of the Savannah, and with that a fight against them. After defeating them you will be charge to interrogate them in jail.