Zeon (Zeno in the English version) is a mamono who looks exactly like Gash Bell, save for his pastier colors and purple eyes. His spells are the same as Gash's, only more powerful; additionally, Zeon does not lose consciousness during the spells and thus knows what he is doing while they are being cast. His cloak also has magical abilities, allowing him to protect himself and his bookkeeper if he gets in trouble, and to teleport himself and his bookkeeper to another location. He is very powerful, malevolent and evil, and bears a strong hatred towards humans but more importantly towards Gash.

Dufaux (Dufort in the English version) is his bookkeeper. He's very silent and reserved, almost to the point of having no emotion, and he has psychic abilities.

Zeon is revealed to be Gash's twin brother and the cause of his amnesia. It is revealed in the manga that their father is the king of the Makai (mamono world): an electric elemental who possessed the ultimate power known as "Baou". When twins were born to him and his wife, the "Baou" power was inherited down to Gash, making them fear for what would happen to him when he finally awakens it, as well as the jealousy of his brother when he found out. To protect both brothers, Gash was sent away to live like a commoner, forced into near servitude by his foster mother and consistently teased and taunted by his classmates at a private school. Meanwhile, Zeon was forced to undertake a harsh, military training. When the silver-haired child found out about both his brother and the "Baou" power, he grew resentful, believing that this twin he never met would hold his father's ultimate ability without understanding the implications. With both children referred for the Mamono Fight, Zeon continued his preparations, training his body as well as reading of certain items and artifacts that would assist him when sent to the human world.

By the time Zeon was sent to take part in the Mamono Fight, he took action in order to guarantee both his own victory and his brother's ultimate misery. After finding his bookkeeper, he tracked down Gash to a forest in England, where he was living peacefully with the local fauna. Without saying who he was, Zeon brutally attacked the golden-haired lookalike, both physically abusing him and using his own spells to smash him down. But instead of burning his book, where he would instantly go home to his family and live peacefully in the makai, he decided to remove his memories to force him to be stuck wandering around this strange, human world without any friends or any idea who he was. Shortly thereafter, Gash was found by Seitaro Takamine, who eventually sent Gash to Japan, so that Seitaro's son, Kiyomaro, could try to find out Gash's origin, as well as to benefit Kiyomaro's personality.

Zeon took interest in the immense tower of Faudo from the moment it first arrived in the human world, having done research on the locked away force within in the libraries of his home. After extensive preparation, he immediately took action, leading on Kiyomaro and Gash towards the tower by use of a doll-like emissary that told them of the danger about to be revealed. However, while using his brother to help awaken the tower for Riou, all of his works was for his own ambition. Once Faudo was resurrected, Zeon challenged Riou in battle, upon which he immediately crushed him, sent him back and took over his control jewel. While he doesn't divert Faudo from its original mission (the destruction of Japan, as instructed by Riou), Zeon awakens the full capability of the beast, giving guardians inside new life and using its power to upgrade many of the mamono previously fighting under Riou. All of these would give the remaining mamono inside one difficult challenge after another as they moved upwards towards the destined showdown with him and Dufaux.

His book was burned by Gash, but under radically different circumstances in the manga than in the anime, although in both versions the book is burned as a result of a struggle between Gash's Bao Zakeruga and Zeon's Jigaridasu Uruzakeruga. He is resentful of Gash to the end in the anime, breaking the jewel that controls Faudo out of spite. In the manga, Zeon becomes aware that Gash had as hard a childhood as he did, and regrets his own foolishness. The jewel breaks due to the strain of their fight. As they are twins, his father original power is divided by two, Zeon gave his power to Gash so that Bao will be completed to help defeat Faudo.

In the original Japanese version, Zeon is voiced by Urara Takano.
In the VIZ Media dub, Zeno is voiced by Debi Derryberry (who also voices Gash)
(Source : Wikipedia)