Kanata Wakamiya (若宮奏多 Wakamiya Kanata) or as he more commonly known as Reiga Giou.
Kanata grew up alongside Yuki at the Asahi Orphanage, where they shared a close bond like that of brothers. Being older, Kanata moved out before the story’s beginning and became independent, something which Yuki admired and wanted to emulate, but he often visited Yuki and asked him to live with him in the future. When he wasn’t spending time with his dear Yuki, Kanata would often do research about a mysterious old book he had had in his possession since he was little: the Book of Raziel.
Kanata frequently asked Yuki to come live with him and appeared to be against Yuki's residence in the Twilight Mansion. He mysteriously appears in Ashley's amusement park and pleads with Yuki yet again to come with him and not put himself in danger, which Yuki declines.
It would later turn out that Kanata is actually the human incarnation of the Giou Clan’s sworn enemy, Reiga Giou, who once was part of their noble clan but turned his back to them and joined the Duras. Reiga is a necromancer of unbelievable strength and knowledge, which is the reason why he doesn’t recover his memories before his body have physically matured - otherwise the immense stress on his soul and body would kill him. While his heart is just as cold as the ice and snow that surrounds him, the fond memories of his life as Kanata together with Yuki still lingers, confusing and frustrating him, creating a complicated internal conflict about whether or not he should kill the Giou Clan’s last hope – the "Light of God."
However, it Story 62 it appears as though he's made up his mind as he flat out tells the Zweilts that although he doesn't care any for humanity he does care for one human in particular, and that being Yuki.
When Kuroto argues back saying that's a lie but Reiga says it again: that he doesn't want Yuki to die, that he wants to protect him. He says he has been having mixed up feelings lately but that now he knows that is what he feels about it. It is also shown in the last episode of the anime that Reiga is unable to kill Yuki due to his memories and fondness of him.
(Yuki) "Kanata-san...If I had agreed to go with you...If I had seen what was in your heart...Would things have been different?"
"When you're sad or hurting, I'll take care of you. I'll stay with you until you can smile again."
"You shouldn't be so trusting. People only betray people."