Yuu  

Death Grave Keeper, ユウ/死の墓キーパー

The young man in question, who has blonde hair and is friends with Shuichiro, turns out to have the power of kings during both the coup d'état and the second Apocalypse outbreak. He kidnaps Inori and uses her void to kill Shu, but Gai ultimately saves him. Gai identifies this individual as "Death Grave Keeper".