Blank slate

Zen's memory has been wiped, and he can't remember if he's a killer or a hero, though he's awakened with the urge to destroy. His unearthly charisma and dark humor draw loyalty from unlikely adversaries like the bounty hunter was set on collecting him who becomes obsessed enough to become his new partner.

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Adapted from descriptions of Blank Slate, manga by Aya Kanno. (I got caught up enough by the reviews at Amazon I ordered it ;-)