Dark Hunter - Houkago no Futari

Keinoshin and Shotaro are students who spend their after school hours as Dark Hunters. Darkness and light both exist in the world, and it is their job to protect the balance between them. Kei and Sho used to be close friends when they were little, but now they are only reluctant partners. At school, they're seen as opposites--the cool and aloof Sho and the cute and friendly Kei. However, they must work together if they want to stop the forces of darkness. Their first case of the manga involves the mysterious vanishing of many cats and dogs in the area, and a second involves a little boy who can see the 'ghost' of an old homeless man he had befriended.

(from mangaupdates)