Varaharu

Varaharu, also known as "The Monster" in Saiyuuki Reload Gunlock anime, is the youkai that has been possessing Hazel for half of his life. The host, Hazel, has been unaware of his own possession. It's from this youkai that Hazel derives his ability to move and exchange souls.

In Saiyuuki Reload Gunlock anime, Varaharu's name was never revealed in the anime, he's simply referred as "The Monster". Varaharu also has a very different origin as well. Varaharu is a youkai (Also known as the monster in the west) that Hazel encountered during his childhood, where he chasing Hazel who'm the latter tried to kill. Since Hazel hated monsters due to his parents being killed by them, the monster cursed Hazel by going inside his body. His master, Filbert Grouse, asked if he is okay despite witnessing his transformation into a monster. The berserk Hazel killed his own master. In his master's final moments, he told Hazel that he needs to let go of his hatred and find a person who will understand him.

When Hazel meet Gat for the first time, Varuharu took over the control which caused Hazel transformed into monster due to the former possessed him before. Gat ended up killing Hazel due to his transformation. The moment when Gato shuts Hazel eyes, he was able to see Hazel's sad memories. At that time, Gat requested the spirit to give his soul to Hazel in attempt to revive him. After Hazel was revived, his past memories and the monster were sealed away by the spirit. The seal would not be broken as long as the spirit is still alive.

Years later, after the spirit had died. the seal inside Hazel's body were broken and then the monster inside him emerged.

(Source: Saiyuuki Wiki)