Fusion

The power of Fusion passes through the Hyuga bloodline. It is the forbidden and thought lost art of fusing one's soul with that of a monster to unlock a new physical form to be used in battle. Yuri inherits this ability from his father, Colonel Ben Hyuga, and awakens to it when the living dead attack his village and kill his mother. Yuri becomes a "Harmonixer" and uses his power to defend himself and occasionally others.

Yuri's power deeply unsettles and scares him. He fears losing his soul to the darkness and killing others in madness. I guess it is to be expected, what with having a Graveyard for the monsters he's slain in his soul. He knows he's cursed and that one day the malice those monsters have for him will overtake him and he will lose his soul entirely. It's the dark fate of a Harmonixer.

Yuri tries to forget his fears by pretending they don't exist. We all know that doesn't work in the real world and in the land of RPGs, it doesn't work and it forces a demon called Foxface to appear from his soul. Sucks to be a hero. Foxface is the embodiment of Yuri's doubts and uncertainly. When he taunts Yuri, he isn't some outsider looking to undermine him; he is that little voice in the back of Yuri's head given shape thanks to the malice of monsters inside of him. For his part, Yuri continues to ignore the voice. After all, he's protecting Alice! If he's strong enough to save her, it's all good, right?

Not quite. Dehuai summons a deity known as the Seraphic Radiance. With no hope of defeating such a creature, Yuri does what is a last and frightening resort. He attempts to fuse with it and loses himself completely to the power of the beast. Shangai is razed and Yuri vanishes for six months. His worst fears have been realized: he can't control his power, he is a monster, and there is no one who can stop him.

But the story goes on and Alice and Zhuzhen eventually track him down with the help of their neighborhood vampire. Yuri has locked himself away in his Graveyard, a place where the world can't hurt him and where he can't hurt the world. It is also a place ruled by Foxface; ruled by his fear. Is the problem that Yuri can't control the Seraphic Radiance or that he is too afraid to? I think it is both but mostly the second. He feels consumed by the Seraphic Radiance and that he has become soulless. He would much rather die than go on living that way.

Alice enters his Graveyard and in the confrontation with Foxface, convinces Yuri to accept the specter for what he is: the other side of Yuri's own soul. His weakness, and his power of fusion, are tied together and inseparable. Yuri's fears don't vanish from that point on, but he doesn't turn his back on them or resent his power. By the time Covenant takes place, Yuri is completely at home with Fusions and no longer harbors his doubts.