Shiro Kanzaki

Shiro Kanzaki (神崎 士郎) is the mysterious antagonist of Kamen Rider Ryuki. Shiro is the creator of both the Mirror Monsters and the Card Decks used by the Riders participating in the Rider Battle, and Yui Kanzaki's older brother. He distributed the decks amongst a wide array of different people and personalities in order to find the Final Rider, who would take on his representative in the Rider Battle, Kamen Rider Odin.

In truth, he's a more of a tragic character, a kind man at heart who was broken by loneliness. He orchestrated the entire war for the sake of saving his sister, and accepted her request of ending the useless war.

History
Shiro started his life badly, constantly abused by his parents and essentially being confined to his attic with his only company, his sister Yui. To ease their sorrows, they would draw worlds where they were happy and free, safe under the protection of Monsters.

However, when Yui was seven, her body couldn't handle the strain of living in such horrible conditions and thus succumbed to death. Broken and afraid of living alone, Shiro found hope when Yui's mirror self agreed to replace her, but on the condition that she would disappear when she became 20. The act of Mirror Yui exiting the Mirror World and merging with her real world self caused an explosion and a fire that killed their parents.

Shiro spent the next thirteen years separated from Yui when he was adopted by American relatives. He thus used this time to theorize and study his own creation of the Mirror World in the hopes of saving his sister. It was from this that he managed to create the Mirror Monsters, the Card Decks and the Mirror World. He then hand-picked Riders that he believed would further the conflict, planning on all of them killing off each other until his own puppet, Odin, would finish off the last. In doing so, he somehow changed himself into a Mirror Being, likely due to merging himself with his Mirror Self to gain more power.

But there was one thing stopping him: Shinji Kido. A Rider who was never chosen by him, Shinji's dogged determination to end the war and stop the fighting prevented deaths from occurring as he befriended Riders and saved them from dying.

All these delays eventually led to the Rider War lasting far longer than it should have, past the day of Yui's death. Shiro would use his "Time Vent" to rewind time back to the time of his sister's death, hoping to prevent it. But each and every time, Shinji would somehow find a way to become Kamen Rider Ryuki, leading to the death of his sister before the War would end.

In the final loop shown in the TV series, Shiro realizes how his obsession to save his sister brought nothing but agony for him, and would only bring pain to his sister should he succeed in sacrificing many lives to save her. Thus, Shiro uses Time Vent one last time, creating a timeline where the Mirror Monsters never existed, and spending eternity living happily with his sister in the Mirror World, drawing pictures of a world where everyone is happy.

Personality
Shiro's an emotionless man, who's only purpose is to save Yui Kanzaki, his sister. His wish to save his sister clouds his vision when it comes to manipulating other people into becoming Riders, knowing full well that the majority won't survive the impending battle. His mind's so clouded, that he used Time Vent at one point simply to save Yui's painting that was ruined by a vase being knocked over. Towards the end of the Rider Battle, the Riders begin to see Shiro's raw emotion in attempting to save his sister, that had remained hidden for a majority of the battle.

His true personality is that of a kind, understanding older brother, always willing to help others. When he finally saw the futility of his neverending quest to save Yui and the horrible sacrifices he would have to make for her to live an unhappy life, he accepted his sister's words.

Trivia

 * He's the only character in the series who is able to go to both the real and Mirror World without needing to transform into a Kamen Rider.