Deker is a mysterious Nighlok swordsman who doesn't remember his past and is cursed into finding the ideal opponent to duel. He is armed with the katana Uramasa and called by Xandred as the Cursed Warrior due to being half-human. Deker was once married to Dayu before becoming half-human. As a human, he was a samurai and was given the blade Uramasa by Dayu. But when a house fire almost kills him, Dayu surrendered her humanity to Serrator to revive him. However this erased his memory and turned him half-human half-Nighlok cursed to forever wander with a thirst for battle. Due to being a half-human, Deker has the ability to assume human form and transverse between the Netherworld and the land of the living without relying on the Sanzu River for survival. He considers the Red Ranger to be his greatest challenge since Uramasa's blade became radiant.
In "Boxed In," Deker claims that the Ultimate Duel might free him from his Nighlok half. Deker is able to remember his past as Dayu's husband. In "The Ultimate Duel," Deker duels with Jayden while the others fought Rhinosnorus. Eventually, Jayden allows Deker to injure him allowing him to get close enough to finish Deker off. Deker's sword Uramasa is broken and half of it gets struck into the ground nearby. Deker thanks Jayden for the Ultimate Duel before vanishing in a puff of smoke while falling off the cliff.
In "Kevin's Choice", Dayu finds a piece of Uramasa as Serrator arrives to prevent Dayu from throwing the broken part away. Dayu later finds Deker alive in the forest. Deker tells Dayu that Uramasa took the damage for him as she tells him about Serrator's offer. With a backup sword, Deker joins Dayu into attacking the Rangers where Deker fights Jayden stating that he is now a "sword for hire" ever since Uramasa broke. Deker and Dayu then attack Skarf in order to unleash Skarf's true power.
In "Trust Me", Deker accompanies Serrator and Dayu to Monalua, where they use a special ash to make its inhabitants paranoid. Deker fights Jayden and Mia, where Deker defeats them and states that Jayden has become weaker for relying on his teammates to help him than fighting him solo. Following Malden's destruction, Jayden uses his Bullzooka when confronted by Deker who asks if Jayden has stated fighting with blasters now. After Jayden destroys the fires spreading the ash and lifting the paranoia spell, Deker was prepared to fight Jayden only for Serrator to tell him that he has bigger plans for him. Deker than leaves with Serrator and Dayu as Serrator summons a Papyrox and two giant Spitfangs to cover their escape.
In the episode "A Crack in the World", Serrator refuses to give back Uramasa straight away. Serrator reveals his plans to Deker and promises him he will be free of his curse if he helps him.
In the following episode "A Stroke of Fate," Deker returns to the ruins of his home as he begins to remember some of his past memories. Dayu tries to convince him not to trust Serrator and Antonio begs him to reconsider his role in the battle between Humans and Nighloks, to no avail. Once Deker reobtains Uramasa, he betrays Serrator and strikes him down, leaving the Rangers to finish him off for good.
In "The Great Duel," Deker challenges Jayden to the Ultimate Duel once more after Jayden leaves the Shiba House. In "Evil Reborn," Deker and Jayden duel on horseback as Jadyen successfully defeats Deker. With both combatants weakened, Deker rises and finds himself too tired to lift Uramasa. Deker then starts hallucinating where he sees Dayu in human form. Deker then charges towards Jayden only for Deker to be taken down by Kevin. Before disintegrating, Deker declares himself free.