Very little of Deker's past is known, but at some point in time he proposed to and married a woman named Dahlia. On their wedding night, he carried her back to their home and gave her with a guitar as a wedding present. Dahlia presented Deker the white Katana "Uramasa", calling him her "brave Samurai", implying that he is a warrior trained in the way of the Bushido. During the night their house caught fire. Deker was nowhere to be found when Dahlia woke to find the bedroom burning, but she managed to find and rescue him from the burning building.
When she called for help, she summoned the King of the Nighloks (later reveled to be Serrator) who offered to save Deker's life. When she agreed to the demon's terms, he did indeed save Deker's life, but robbed him of all his memories and turned Deker into a Human/Nighlok hybrid and Dahlia into the Nighlok Dayu in the process. Revived from near death, Deker transformed into his Nighlok half and vanished into the night, leaving Dahlia behind. The of events of that night was eventually made into a folktale told in the form of a painting at the Tengen Gate.
He remains oblivious to his connection to Dayu despite rescuing her from the Red Ranger's Fire Smasher in a battle against the Rangers and interacting with her general. When she asks about his life, he tells he remembers nothing beyond needing to fight, that nothing else is important. Dayu is later seen to regret her decision to make the deal with the Nighlok King to save his life, heartbroken over what it has done to her husband. Like Dayu, Deker's unique position as a Human/Nighlook allows him to leave the Netherworld whenever he wishes without needing to return to the Sanzu River like the other Nighloks and Master Xandred. As he is not apart of Master Xandred's group, the two are enemies. Xandred despises him for his half-human state and constant meddling in his plans to destroy the Samurai Rangers, once misunderstanding his intervention as "helping" the humans despite Deker's protests to otherwise.
As a cursed man with the unquenchable desire to fight, Deker roamed the world for a worthy opponent to battle. Uramasa eventually leads him to Jayden, the Red Samurai Ranger. Deker is determined to fight the Ranger one-on-one to the point of single-mindedness, often appearing at inopportune times or being halted by the other Rangers and monsters when the opportunity strikes him as right.
When Jayden is poisoned by Octoroo, Deker kidnaps him and dumps him into the holy springs to cure him as a means to prepare him for the fight. While Jayden is recovering, Deker tells him that he has little choice in the matter against his need for fighting, though Jayden argues that "there's always a choice". Antonio eventually arrives and rescues Jayden from Deker, who promises there would nothing to stop their fight the next time they meet.
When the two finally meet again, he gave Jayden the choice of fighting him or being forced to watch as he harmed innocent people if he refused. It's later revealed that his desire for battling Jayden is fueled by the belief that it will free him of his Nighlok half. He finally battled Jayden and appeared to have the upper hand in the fight when he stabbed the Red Ranger in the right side of his chest. Though he was stunned, Jayden allowed the warrior to run him through and proceeded struck Deker down in one slash. Uramasa was shattered, broken in two. Surprised by the Red Ranger's audacity, Deker complimented Jayden on his win and stumbling towards the edge of the cliff, he fell over the edge and disappeared in a puff of purple flames. Deker was not seen again after The Ultimate Duel. After that, he is not seen again, until "Kevin's Choice". Dayu talks with him about fixing her Harmonium, as well as his Uramasa. She further explains that if they make a deal with Serrator, that Serrator will fix their respective items.
(From the Power Rangers Wiki)