Juujika no Rokunin

Overview

In order to defeat several non-human monsters, the boy also became something non-human. Shin Uruma, a sixth-grade student, was named "Experimental body: A" by five of his classmates and was severely bullied and lived in hell. His only peace was with his brother, who loved him, and his parents, who protected him.... . until the five monsters killed his family. When he finally loses everything and faces true hell, a dark "wish" is born inside Shun. He was reborn under the training of his grandfather, who served in a secret unit during WWII. Now, four years later, he appears in front of his fated enemy. "I won't let ANYONE get in the way of my revenge." Note: Includes two extra chapters.

Source: AniList