Shinji Kirihara

A boy consumed by a possessed sword filled to the brim with pitch black hatred and once the successor to the Kirihara Family lineage: Shinji Kirihara. Initially, he was a good kid that cared for his father and twin brother, Reiji. He was an exceptional swordsman and showed great prowess with a blade. When Shinji preformed the ritual to become the family head, he was consumed by the demon sword: Yamakaze. The blade is possessed by a mononoke, though it has lost its sense of self, and only an overflowing aura of hatred and power lingers. Now, he waits for the day where he and Reiji clash their blades to the end.

Growing up, Shinji was a nice kid that cared for his brother immensely. One time, he cut Reiji while they were dueling and he immediately went to see if he was okay. Whenever he was praised, he was very thankful and excited as well. After being consumed by Yamakaze, Shinji became a hateful person that wishes to pay back those that wronged him, even if they weren't of bad intent. Due to this, he joined Amagi's cause, but only to achieve his own goals. In actuality, Shinji was once a worse swordsman than Reiji was, when they were children. After his father hit him for being the eldest yet being beaten, Reiji unconsciously held back and was always one step behind Shinji, letting him always win after that.

As this happened, Shinji was filled with despair, knowing that his brother was just one step back from being at his level. It's unknown if he hated Reiji and his father or not prior to the ritual, though once Yamakaze was in his grasp, the hatred overflowed into him and amplified his feelings. In a way, it made Shinji show his true self. Once that happened, he had swiftly sliced off his father's head, in tears, and told Reiji to become stronger to kill him. Shinji and Reiji eventually do have a reunion when Shinji is assigned as Naruko's bodyguard. Reiji is in disbelief and ultimately gets seriously injured, though Shinji spares him to let him process everything he learned of his despair.