3000 years ago, Merrick donned a wolf mask, transforming him into the evil Zen-Aku. Zen-Aku battled the original Wild Force, and was defeated and imprisoned. He awakened in present day, and sought revenge on the successors of the Wild Force powers. Zen-Aku's memory was cloudy- he did not remember his true identity, and did not clearly recall the events which led up to his imprisonment, thus making his motives unclear at times. He was eventually broken free of the spell, and was reverted to Merrick, who then became the Lunar Wolf Ranger.

The mask of Zen-Aku eventualy reformed, and Zen-Aku was brought back to life, no longer needing a host to live in this world. He went after Merrick to try and merge with him and regain the power they had as one. Merrick was victorious, and destroyed Zen-Aku again. After the final battle againstMaster Orgwas over, Merrick left totravelthe world. Surprisingly, he was approached by Zen-Aku, who had once again come back to life. This time however, he was not out to do evil. He sought redemption for his evil deeds, just like Merrick, and joined him on his journeys across the globe.