Bonding to Merrick
Not much is known about Zen-Aku's origins, except that his spirit was locked inside a cursed wolf mask. After the defeat of Animus at the hands of Master Org, Merrick decided that his only hope of stopping him was a legend about a cursed mask that could bestow great power upon the wearer. He found his way into the lair of the Org General, Nayzor, where he found the mask of Zen-Aku. Merrick put it on and gained its powers, using the Predazord to destroy Master Org, but immediately afterwards, the wolf mask seized control of his mind, merging him with and transforming him into Zen-Aku.
With what humanity he had left, Merrick begged the other guardians of the Animarium to destroy him, but they refused to destroy their friend. Instead the guardians sealed Zen-Aku in an underground tomb, where he remained for 3000 years.
However 3000 years later, Zen-Aku was released by Nayzor to destroy the present day Animaria Guardians, the Wild Force Power Rangers. He possessed no memory of his past, only that he wanted to destroy the rangers in vengeance for their predecessors sealing him. Zen-Aku proved able to fight the rangers quite well, even using his own Wildzords, and succeeded in stealing the Elephant, Giraffe, Black Bear, and Polar Bear Animal Crystals and adding them to his collection of Dark Wildzords. However as time went by, Zen-Aku's past nature began revealing itself. Once he found the White Ranger, Alyssa, injured at the hands of Toxica, and bandaged her up instead of finishing her.
Another time, Zen-Aku saved a wolf/dog pup when it was in danger from his fellow Orgs, even attacking Toxica to protect it. He began to become confused, even more so when he started to have flashbacks of his past. Zen-Aku began to doubt himself even more when the spirit of Animus began to appear before him, telling him to "Remember", which confused the ranger as well, especially after Princess Shayla saw Zen-Aku bleeding (which Orgs are unable to do) and they saw his actions. As he came even closer and closer to realizing the truth, Nayzor eventually used an insect that took possession of the wolf Org's mind and made him into his willing servant.
However Princess Shayla had begun to piece together the truth and later, Animus managed to cause a lunar eclipse that allowed Merrick to take back control. He told the rangers' what had happened to him, but then the eclipse ended, changing him back into Zen-Aku. Shayla managed to piece everything together and realized that the curse came from the Predazord, because after Master Org's defeat, the wolf mask had made the Wolf, Hammerhead, and Alligator Wildzord's evil and their guardian along with them. Shayla then told the rangers that the only way to free Merrick was to defeat the Predazord, which they succeeded in doing with help from the new Armadillo and Rhino Wildzords.
However Merrick and Zen-Aku had unfinished business with each other and thanks to the mask being broken, the Wolf Org returned, this time determined to remerge with Merrick. Now he was even stronger then the last time, which Shayla explained was because when they had been bonded, Zen-Aku's evil power had been divided between Merrick, Zen-Aku, and the Predazord, but with Merrick and the Predazord good again, the Wolf Org had all his power to himself. Zen-Aku battled Merrick and almost succeeded in his goal, but was eventually defeated again with use of the Predazord's Revolver Phantom attack.
Traveling with Merrick
After Master Org was defeated again, Merrick set out to wander the world and was approached by Zen-Aku, who offered to join him, saying that they were both "lone wolves looking for redemption". Merrick accepted the offer and the two set off together.
Zen-Aku is quite powerful, able to take on all five of the rangers in close combat without too much difficulty. In battle, he wields a large crescent shaped blade that he can use to send out crescent shaped waves of energy. When Zen-Aku was bonded with Merrick, he played a knife shaped flute to summon the Dark Wildzords and could use his powers to corrupt Animal Crystals, in order to add the Wildzords they represent to his collection of Dark Wildzords. Zen-Aku's powers wax and wane with the moon, at the full moon he is at his strongest and when there is no moon, his powers were diminished enough to allow Merrick to take control. Zen-Aku can also grow to giant size, as he demonstrated after being separated from Merrick and losing the Dark Wildzords.
- Zen-Aku's manner of summoning his Dark Wildzords by playing on his flute is similar to how Tommy Oliver summoned the Dragonzord by playing on his Dragon Dagger like a flute.
- Zen-Aku's name means "good-evil" in Japanese.
- When the Predazord was under Zen-Aku's control, its head resembled him.
- Until episode "Unfinished Business," Rangers believed that Zen-Aku was simply Merrick's evil self as result of being exposed by the dark power within the cursed wolf mask. When Zen-Aku returned as a full-Org and went after Merrick, they finally learned that Merrick and Zen-Aku were separate entities and thus, how Merrick and Zen-Aku's relationship was more like possession.