"Berserk Evil Dragon" Zero「"狂邪竜"ゼロ, "KyouJaryuu" Zero」is an artificial living being created by Yuuki Kagurazaka who was accepted into the Eastern Empire's Mixed Corps. After Yuuki unleashed Armageddon, he reincarnated as a Seraphim Executioner and later, after being given the Ultimate Skill Azi Dahaka and successfully taking control of Ramiris' Labyrinth, he fused with the skill and together evolved into the 'sixth' True Dragon known as "Berserk Evil Dragon."
Unlike Vega, who was given an aggressive personality, he wasn't given personality but in exchange for that his Calculation is far superior than Vega, thus he could be considered as Yuuki's Ultimate Creation.
He was one of Yuuki's 300 elite subordinates
Ryuma Clash Arc
It's mentioned that he and the other 300 Yuuki's Elite Subordinate were gathered in Manor at Eastern Empire Capital under order from Yuuki in order to discuss about Coup d'état Plan after that he was Purged By Kondou, Damrada, Miraza, and the Royal Knights No.7 - 10.
Tenma Great War Arc
During the attack on Tempest, Zero consumed the Labyrinth using Evil Dragon Lord Azi Dahaka. Because of the large energy from the Labyrinth, Zero lost control of his body to the Ultimate Skill and evolved into True Dragon, Evil Dragon Zero. He later was killed by Diablo, Benimaru, Zegion and Dino's combined attack and his skill, Evil Dragon Lord Azi Dahaka reduce to become a dragon core which later combined with Gaia and evolved him into a True Dragon.
As one of Yuuki's Top Subordinate his abilities is far above Average Royal Knight.