Zess-Luka Crosszeria

A young man with silver eyes and black hair who's been protecting Yuki from the shadows ever since his birth 15 years ago. Despite being a duras (a.k.a demon), he's currently acting on Giou Clan 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, his facial expressions softens when he's with Yuki. 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 in Yuki's previous life. This is yet to be explained. It was during the last war that he changed side to the Giou Clan, as he apparently made a binding "contract" with Yuki. Luka is a descendant of the "family of sinners" (Zess) and a high-grade demon (Opasts). Luka battles with a large black sword and can use powerful magic (and even summon a powerful Salamander dragon). He was once the loyal dog for the demon king and used to drink his blood, that's why he is stronger than any opasts and it's possible that he is stronger than the demon king himself. But he left his position as the loyal dog of the demon king after meeting Yuki. It was during the previous battle that Luka met Yuki and they fell in love. Luka promises he will stay with Yuki as long as Yuki wants him to.
The manga suggests a bit of a darker history. Luka tells Yuki he was considered a slave because he belonged to the "sinner" clan, Crosszeria, 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, put his own blood into Luka's brand. The demon king's blood made the brand 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 blood seal, 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 voided, allowing Luka to live. Luka hints that the prior contract with the demon king may not be "completely" voided, however.