Luze (ルゼ Ruze) is Luka's younger twin brother. Unlike Luka, he has amethyst-colored eyes, and his hair is longer in length and is worn in a ponytail. In the manga, he seems to relate with Reiga/Kanata in the fact that he is also half Duras, and he harbors feelings of resentment towards Luka because he betrayed the Duras and further soiled the Crosszeria Clan.
He is Reiga's right hand. Reiga also taught him the Necromancy ability, which is why Luze called him by "Fawler".
When the Crosszeria twins were younger they liked a lot of the same things but as Luka was taken on as the Brand Zess, they started going in different directions. Luka was training constantly and running errands for the Demon King, while Luze was allowed to grow up and do much of what he wanted.
Luze secretly wanted to catch up to his twin brother, so he worked to become a first general class Opast in Infernus.
However, Luka Crosszeria or "Zess" became a traitor and abandon his clan. Luze, who was on duty, heard this fact directly facing the Demon King. He eventually signed to Reiga and was summoned to earth.
Luze doesn't seem to care or have much of an opinion on Yuki but is concerned with the relationships involving him, such as his brother betraying the clan for him and Reiga's lingering feelings for Yuki.
At the end of the series, Reiga's last order to Luze, after releasing him from his contract, was to protect Yuki after the war was over, which Luze did, joining Luka.