Midnight Sun

About two boys named Shitou Teruaki and Ayatsuji Kaoru. Teruaki is a top-notch bounty hunter as is Kaoru. The story starts off with Teruaki being given a mission to track down a serial rapist and he unwittingly meets up with his rival "K"--also known as Ayatsuji Kaoru, and who also happens to be the most popular boy in the school he just transferred to--by "saving" Kaoru from being another victim of the rapist.

(from Aestheticism.com)