Zero (ゼロ, Zero) is a lone wolf who trained Jio.
Zero is a lone wolf who has been banished by other wolves because he is an OPT. He taught Jio all he could of fighting and survival skills. He hates both humans and wolves, but built up a friendship with Jio as they share similar situations - they are both shunned by others of their species.
Zero was first seen when he saved Jio from a sand worm. Jio was afraid of him in the beginning, since he thought that Zero would eat him, but Zero told him that he did not eat humans, and was also surprised that Jio could understand him. Jio started begging him to teach him how to be strong like him, but Zero originally refused, since he did not want to be friends with a human, and that he saved him only because he did not want that worm to hunt on his territory. But he eventually agreed as Jio continued to ask and he saw something in his eyes.
One night, Jio asked him why he is now howling like the other wolves, to which Zero replied that this is a sign among friends and has nothing to do with a lone wolf like him.
One year later, Jio's training started to show that he was becoming much stronger and faster. One night, Jio proposed that they catch that a worm and eat it. However, Zero told him to not do it, since the worm is strong. But on the next day, Jio saw the worm and decided to attack him and when he was about to be eaten by it, Zero came and pushed him out of the way and getting him by the worm. Seeing the worm getting angry, and faster, he was forced to activate his effect of the O-Part, something he was avoiding of doing, since it brings great pain to him. Being now much faster, he used his speed to keep the worm occupied while Jio runs. Soon after that, he could barely move his body, due to the pain and he surrendered to the worm, saying to eat him and that he won't move. While waiting for the worm to eat him, and thinking that he just saved a human kid, he realized what his dream is. A moment before the worm eat him, Jio came flying from above with a giant rock hitting the worm and making it spit all kinds of things he have eaten. But the worm still stood up to their surprise, Jio grabbed a sword that the worm spit out and throw it at him, but it didn't even scratch him. He then found a wooden boomerang, and Zero told him to just throw it to divert his attention while they try to escape. While Jio was preparing to throw the boomerang, he released his spirit and activated the boomerang's effect, and Zero realized that Jio was just like him, an O.P.T. After beating the worm with the boomerang, Jio and Zero was later seen eating the meat, where Jio commented he is tired of eating that meat and wanted to catch something else, but Zero told him to leave. Zero told him that he is now much stronger and to leave to fulfil his dream, and chased him out. While running Jio, heard a wolf's howl, realizing it was from Zero and he howled after him. Hearing that Zero commented that he finally found a fine looking fang and to look around the world to strive for his dream.
While Zero itself is not an O-Part, and the exact cause is not know, something seems to have happened to his father long ago and now his bones turned into an O-Part. His O-Part bones are Rank C and his effect is high-speed movement, thus when he active his O-Part effect, he gains tremendous speed, but that brings pain to his body, so he rarely uses it.
In post time-skip, he travels with Jio. It has been revealed the his father is the No. 4 Angel - Zadkiel and then he obtained his power.
His special power allows sacrificial wolves to take on the shape of another person and save them from an attack. The cloned wolf will die in place of the replaced person. The person will still feel the pain of their death ex; gunshots, laser beams, etc. He was sucked into the completed Kabbalah inside Shin which help start Armageddon which was stopped as Jio overcame Satan. Inside Shin, to save the world with the help of Jio, Ruby, Cross, and Jack himself, they warped shin away from Earth just as night fell disappearing into the void of space taking the effects of Armageddon with them and saving the Earth.