|First Appearance||Episode 1\br|
|Race||Duras AKA Demon|
|Family||Luze (younger twin brother)|
|Voiced by||Sakurai Takahiro|
Zess (Luka Crosszeria) (ゼス(ルカ＝クロスゼリア), is a general class opast who's been protecting Yuki from the shadows ever since his birth 15 years ago. Despite being a Duras (also called demon;youkai; ayakashi; etc.), he's currently acting on Giou Clan's side and is recognized as a traitor by the other Duras. Normally reserved and indifferent, his emotions are more expressed when he's with Yuki. Luka is a member of the Crosszeria Clan (also known as the "Clan of the Traitor") and a high-level duras (an opast). Luka battles with a large black sword called ROXASS and can use powerful magic (which he can use to summon his powerful dragon familiar, Sodom). He was once the beloved slave of the demon king and had a blood covenant with him.
Luka Crosszeria is calm and constantly cool, even in a fight, and is pretty much indifferent to everyone but Yuki, to whom he is devoted. Although he is usually so balanced, he can suddenly grow furious, and his power flies up to levels that can be felt from far away. He doesn't trust anyone very much and likes to do things by himself, as towards the end of the anime, where he almost set off to fight Reiga (who captured Yuki) and his subordinates alone. He fell deeply in love with the previous Yuki (who was a girl), even betraying his family. Although Yuki doesn't recall the details of his previous life, Luka still loves her.
"His eyes are blades, and his hair the night. His shape and form are frightening and yet beautiful."
A very handsome man, Luka has pale skin, medium length black hair and silver eyes that Yuki describes as being "not of this world". He is tall with a medium build and has a red "brand zess" on his left shoulder. He also has a small claw shaped earring on his left ear which allows him to call on his weapon (much like the rings that the Zweilt use to call their demonbane weapons); ROXASS, a black sword. He also wears another earring on the helix of his left ear that's engraved with a control spell. He often dresses in a black coat which overflows to his feet, and has black cross around his neck, which was made and given to him by Yuki. He has also long fingernails, and often uses many different kinds of belts.
Luka tells Yuki he was considered a slave because he was born to the Crosszeria clan (also known as the "Clan of the Traitor"), and that all the other Duras were intent on punishing the whole family and all its descendants for the crime of selling the names of 'his' brethen over to humans. For a duras, having your name known is the same as being enslaved. The demon king owned Luka, and as a symbol of this, he formed a permanent covenant by putting his own blood into Luka's Brand Zess, dying it red and causing Luka a great deal of pain during the process. Years later, Luka is still haunted by the memory and feels physically ill when recalling it. Because of the covenant, if Luka should ever betray the demon king, he would die, but as he made a new contract with Yuki, the previous contract with the demon king was forcefully nullified. However, Luka hints that the prior contract with the demon king may not be completely nullified as long as his arm carries the "bloody cross".
His relationship with Yuki is complicated. He is completely dedicated to Yuki considering him more important than anything else in the world. Yet he won't tell him of their relationship Yuki 's previous life (this is yet to be explained in the manga). It was during the last war that he change sides to the Giou Clan, as he apparently made contract/covenant with Yuki's previous life and had it renewed by force with present Yuki. But he left his position as the loyal servant of the demon king after meeting the present Yuki's previous life. It was during the previous battle that Luka met Yuki (when Yuki was still a girl) and they fell in love. Luka promises he will stay with Yuki as long as Yuki wants to.
Luka is wary of Takashiro and finds him unworthy of his trust, despite Takashiro's interest in protecting Yuki at all costs. When Yuki's well-being was being discussed outside the orphanage, Luka attacked Takashiro, just barely missing him on purpose. This is a clear example of the amount of agitation Takashiro causes Luka. A scene that was omitted from the anime, but is present in the manga is when Luka says "You piss me off" to Takashiro and then storms off.
Luka has a friendly relationship with Toko. Although he called her a "random girl" at first, not even telling her his real name, their friendship eventually strengthens. Toko often teases Luka, which probably annoys him.
Luze is Luka's younger twin brother. They have no affection for each other and have only met a few times. Luze considers Luka to be a traitor that dishonered their family, and is willing to kill him to help achieve Reiga's goal. The brothers look almost exactly alike, the only differences being that Luze has longer hair and amethyst eyes.