Zen-Aku

Zen-Aku is a Wolf Duke Org who had manifested from the Guardian Merrick Baliton, when the latter needed the former's mask to defeat the original Master Org 3000 years ago.

Biography
No one knows much of Zen-Aku's origin, only that his Org spirit was locked within an evil Wolf Mask. When Merrick's friend Animus had perished at the hands of the original Master Org, Merrick made the decision that he had to stop their enemy once and for all. Heading into a temple, he'd found the Mask of Zen-Aku, put it on, and used his Predazord to destroy the evil Master once and for all.

However, after Master Org was defeated, the Wolf Mask had seized control of Merrick's mind, turning him into (or merging him with) the evil Zen-Aku. While he still had some of his humanity, he implored his fellow Animarian Guardians to destroy him, but they refused and instead used their Crystal Sabers to seal him away in an underground coffin.

"Curse of the Wolf"
For 3000 years, Merrick, now Zen-Aku, remained in stasis, until the Master Org of the present-day summoned a General Org named Nayzor, who released the Wolf Duke Org on the present-day Guardians of Animaria—the Wild Force Rangers. However, he had not had any memory of his former life, only that he knew that he wanted revenge on the Guardians who sealed him away 3000 years ago, and he tried to take it by capturing the Rangers' Wild Zords and turning those he captured against their chosen ones.

Zen-Aku also revealed that his power came from the phases of the moon, as he was at his strongest when it was full, but during the New Moon's phase, he was revealed to be human (in a flashback scene where Cole saw a young man in the river, accompanied by the wolf cub Zen-Aku had rescued).

Zen-Aku was able to control his own Wild Zords, which he called the "Dark Wild Zords", using his flute plate to summon them (much like Tommy, the Green Ranger used his Dragon Dagger to summon Dragonzord). He could also combine them to form the Predazord.

Zen-Aku's main weapon was his "Crescent Blade", whose blade could open at two notches. While in that position, his weapon's main attack was the "Crescent Wave", which was a blade of dark energy that could disable almost any of his opponents.

Meeting Animus
Over time, Zen-Aku's true nature had been revealed, as he began slowly acting more and more human, by starting out with caring for Alyssa, the White Wild Force Ranger, and rescuing a wolf cub from an attack from another Org. The Rangers began to grow confused, and even more so, when the spirit of Animus appeared in the sky and urged Zen-Aku to simply "Remember." However, just as his human self (Merrick) was about to break free of Zen-Aku's curse, Nayzor arrived and turned him into his willing slave, making him unable to be reasoned with.

"The Ancient Warrior"
When Princess Shayla learned that Zen-Aku was really her faithful Guardian Merrick, she tried to piece together the events of the final battle in Animaria. She discovered that the source of the curse put on Merrick was not the Wolf Mask, but his Predazord. After Master Org was defeated, Predazord had turned evil, and its chosen Guardian along with it. She informed the Rangers that defeating Predazord was the only way to break Merrick's "Curse of the Wolf". With a new Wild Force Megazord combination, they did so, and reverted Merrick to his human form.

"Unfinished Business"
Merrick thought Zen-Aku was gone forever when his Predazord was purified of the evil that poisoned it, but, a few episodes later, Zen-Aku returned with an agenda: to remerge with Merrick and become complete once again. With the return of the Wolf Duke Org, the Rangers began to question Princess Shayla about this event. Princess Shayla told the Rangers that when Zen-Aku's spirit merged with Merrick, it was divided among the Mask of Zen-Aku, Merrick, and his Predazord, but with Predazord and Merrick now good again, Zen-Aku possessed all the evil power that was once divided. Eventually, he was destroyed by Merrick's Predazord's "Revolver Phantom" attack.

Final Appearance
After Master Org was defeated, Merrick traveled the world, and Zen-Aku returned, offering to join him, since they were both lost souls trying to find themselves.