Light Yagami is the mass murderer Kira or, as he likes to call it, God of the New World. His main targets are criminals, as he believes they're scum that do not deserve to live. His main weapon is his Death Note, which can kill by writing down your targets name in its pages.
Light claims that he is doing a good thing for society, and anyone who stands in his way is just as bad as his criminal victims. He will do anything to keep his identity hidden, and to ensure his goals to come to fruition.
