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So here are the facts:
-Kenpachi is an undead. Stating the obvious I know since this is a series about shinigami, which techinically count as undead, but just bear with me.
-Kenpachi seems to work differently than other shinigami, possibly hinting that he's a different kind of undead.
-Kenpachi inflicts negative levels on himself as a means to make his fights more challenging.
-Kenpachi supposedly does this all for self-gratification.
Conclusion:
Kenpachi is a wight with a subconscious chronic masturbation problem.