Doku is the Lord of the Greater Fiends of the Archfiend, Vigoor, and is the secondary antagonist of Ninja Gaiden, as well as a minor villain from Ninja Gaiden Dragon Sword.
Doku was responsible for transforming Rachel's sister, Alma, into a Greater Fiend, which resulted in Rachel having a grudge against him. Some time later, he was ordered by Vigoor to obtain the Dark Dragon Blade from the Hayabusa Village and led an army to assault it. Doku slaughtered many Hayabusa ninjas and obtained the dark blade, before finding himself confronted by a vengeful Ryu Hayabusa. However the Greater Fiend was able to overwhelm the ninja, dealing him a seemingly fatal blow before returning to Tairon, capital of the Holy Vigoorian Empire, and presenting the blade to Vigoor. However Ryu survived and pursued him, fighting his way through Tairon and met Rachel, who told him about Doku, namely that killing him was impossible and that she had a grudge against him. This didn't daunt the Dragon Ninja, who later fought and defeated Alma, though Rachel, who arrived on the scene, couldn't kill her.
Doku confronted Rachel and captured her so as to use her in Awakening Alma, but Ryu caught up with him and the two battled again. This time however, the Greater Fiend was defeated, but not before telling the Dragon Ninja that the Dark Dragon Blade was in the possession of the Vigoor Emperor and that the Dragon Lineage drew the power in their blood from the same deities that the fiends did. However Doku was not done yet and returned in spirit form, brinding Rachel to the Pyramid to begin Alma's Fiendish Awakening.
Ryu arrived to try and stop it, but the Awakened Alma attacked him, only to be defeated and the ritual foiled when she managed to regain some of her humanity to prevent Doku from completing it. He retreated and he and Ryu had their final encounter in the Zarkhan Labyrinth, where he was defeated again, but as he descended into the Underworld, Doku used the last of his power to curse Ryu, transforming into the thing he hated most, a fiend. Later in Ninja Gaiden Dragon Sword, Doku was resurrected by the Dark Dragon Stones, only to be defeated again and his soul catapulted into oblivion.
Doku is rather like a dark samurai, a skilled warrior and extremely loyal to his master, Vigoor, as well as utterly merciless in his missions. When Vigoor gives him a command, he performs it without question and has no regard for anyone that gets in his way. Doku also enjoys mentally tormenting his foes, such as with Rachel when he used Alma against her and telling Ryu about the Dragon Lineage's history.
Doku is quite powerful and wields the dark katana, Kisetsu, except when he fought Ryu the first time, instead using the Dark Dragon Blade. He can generate energy pulses from the sword and can breath a blast of fire-like energy from his mouth. Doku can also use a dashing slash attack, drain life from his opponent, and throw his sword like a boomerang. He also has spiritual powers as seen with his spirit form, where he has most of the same abilities as his normal form except for he can send out his arm to grab his foe, even from a distance.
- In Ninja Gaiden Black, Doku is assisted by enemies during his second and third boss battles.
- He bears striking resemblance to Darth Vader, they both have a deep, robotic voice and are tall with dark armor. Also Vader almost always wears his helmet, never showing his face, while Doku has no face, just purple flame and a black samurai face mask.
- Doku (Kanji: 毒) in Japanese means "poison".