Zen was a transferred person who reincarnated into the Death March world over 40 years before the start of the story. He became infamous as the powerful undead magician known as the No-Life King who destroyed the Muno Marquisate.
Having died powerless and miserable in his previous life, Zen wished to god for a body that won't die, a life without starvation, and power to strike back at unreasonable violence. He was reborn as a healthy baby, raised by good parents, and met a pure lovely spouse. He resurrected in his immortal body after being executed by a noble who had fallen in love with his wife. The first view he saw after being resurrected was the head of his whole family starting from his parents in front of the castle, with the broken doll-like corpse of his wife underneath them. Starting with the corpses of his family, he raised an undead hoard and marched against the noble in revenge, laying waste to everything in his path.
With his revenge now complete, Zen wanted to die in order to join his wife in the next world, but was unable to do so even with a purification technique and holy sword. He captured Mia in order to unseal the Maze of Trazayuya, from which he hope to personally train a hero who could end his life.
Zen was always seen wearing a black robe with a hood and long sleeves. As an undead, it was possibly a precaution to hide his eerie face and skeleton body. He had a bleached bone face.
No-Life King Zen
After failing to capture Mia using his flying ants, he appears before her who is under the protection of Satou and his companions. He forcefully requests that they hand Mia to him then speaking inexplicably to Satou that if he wants to save Mia he should bring a hero along to the maze he currently governs. In the ensuing battle of attrition, Zen finally captures Mia, using the magic, Shadow Portal, but he's surprised when he discovers that Satou had entered the portal in pursuit of saving her.
Mia is next seen--by Satou--sleep-sitting on the throne in the master's room of the maze, she had been brought back to, with an unknown beautiful blond woman nursing her. Zen, whilst being surprised by Satou's presence, deems it unfair for Satou be here yet, as only those who have conquered the maze can enter the master's room then with the conquering 'hero' being the most qualified in annihilating him unable to make sense of his ominous words, Zen activates an impromptu trap floor that falls Satou to the first level of the maze before he can reunite with Mia.
Two purple lights emerged from his crumbled body after being killed by Satou.
- [Shadow Whip]
- [Shadow Portal]
- [Limit Break]WN 5-11, strengthen its recipients to double their level
- [Immortal]WN 5-12, body that won't die, a live without starvation
- [Physical Attack Invalid]WN 5-14, because of those two skills that he wasn't able to kill himself
- [Instant Recovery]WN 5-14, because of those two skills that he wasn't able to kill himself
- Black robe, hood and long sleeves
- Holy Sword Gjallarhorn (gave it to Satou)
- The play which Satou and Zena watches in Seryuu City was likely based on Zen's first death and the execution and suicide of his family and wife respectively.
- Due to the tragedy he experienced in his lives, Zen harbored an intense hatred of nobles and would wreck havoc in the Kingdom for decades in revenge. Zen's hatred for nobles is also evident in his servants such as Nana, who was programmed to treat nobles as primary elimination targets.
- Zen was reincarnated in the world of Death March kara Hajimaru Isekai Kyusoukyoku seemingly from ancient Japan, which he referred to as the land of the god.