A squad leader of the Undertakers, Tamaki's special enforcement group which selects prisoners who could not be rehabilitated and survived a special 'correctional' program to be anti-Deadman soldiers, wielding weapons that allow them to nullify the Deadmen's Branches of Sin. He dresses like a monk (though he prefers to be called a "Hyper Monk") and wields a guitar that can split into two machine guns, fire shock waves, or just play music like a regular slide guitar.
2 years ago he killed Nagi's wife after she conspired with her husband to rig their fight, and has taken an interest in Nagi since then, claiming Nagi possesses the insanity befitting of an Undertaker. During the flashback in episode 12 Genkaku's past is revealed, describing the events that led up to his current insanity. As a young boy Genkaku was a monk at a monastery, where at first he was a quiet, gentle person. However after taking constant abuse at the hands of three other men present at the monastery (it can be assumed from the flashback that it was not only physical, but sexual abuse), coupled with the aftermath of the explosion in Tokyo, he snapped, killing all three of the men.