Yami Kariudo

The manga centers on Takeshi Hazama, who on the surface is a high school student aiming to be a manga creator, but in truth works as a "Yami Kariudo," a Dark Hunter, an assassin who works on behalf of victims and the bereaved to slay evil. As a Dark Hunter, he wields a stainless steel ruler and pen as his weapons. His constant rival includes fellow Dark Hunter, Kyousuke Garyuu, a Treasure Hunter who is at different times an ally or enemy to Takeshi.

(Source: ANN)

Included one-shot:
Volume 5: Hajime no Ippo!