|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 with silver eyes and raven black hair 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. Although his real age is unknown, his appearance looks like that of a college student. Normally reserved and indifferent, although 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 loyal servant of the demon king and had a blood covenant with him, that's why he is stronger than any opasts and it's possible that he is stronger than the demon king himself.
Luka Crosszeria is very calm when he is around Yuki. He doesn't usually get mad unless someone harms Yuki or acts very stupid. Luka is constantly cool, calm and is popular with girls because since he's a general class opast, his appearence is more attractive to humans. Luka will do whatever Yuki says - as he loves Yuki very much (his previous life-when Yuki was a girl). When Luka said "I will not betray you." to Yuki, he meant that he cares deeply for her. Although Yuki doesn't recall details of his previous life, Luka still loves her. In episode 23, Luka's power was suddenly getting furious, when Reiga kidnapped Yuki.
Luka has pale skin, medium length black hair that usually covers the left side of his face, and silver eyes with long lashes. He is tall and has about a medium build, as the manga seems to portray him as having a subtle eight-pack. He also has a small claw shaped earring on his left ear which allows him to call on his main form of 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, as well as dark clothing and a black cross around his neck, which was made and given to him by Yuki. He has also long fingernails, and often uses many different kind of belts.
The manga suggests a bit of a darker history. Luka tells Yuki he was considered a slave because he belonged 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. 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. The demon king's blood made the mark on Luka's arm glow red, and it caused 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 it was a covenant, if Luka should ever betray the demon king, he would die. Luka explains to the current incarnation of Yuki that since he made a new contract with Yuki, the previous contract with the demon king was nullified, allowing Luka to live. However, Luka hints that the prior contract with the demon king may not be completely nullified.
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/convenant 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 barelying 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's relationship with Tooko is one-sided at best. While it has been hinted that Tooko shows affection toward Luka, he does not return the feelings. Mostly though, their interactions are kept on a friendly level with Tooko often teasing him which most likely annoys him like when she found out his true name was "Luka".
Luze is Luka's younger twin brother. However, since Luka betrayed his clan to side with humans, Luze thinks of him as a traitor. They don't seem to get along very well and Luka has said that even though they are siblings they have met only a few times. Luze looks exactly like Luka except that he has long hair and his eye colour is amethyst.