She eventually grew to like Ren and, although she never quite got what sort of relationship the trio has, stopped thinking it was something naughty.
The parents of Yui and Shiro were, I think, terrible people. We've seen that they locked their children in a room in the house. Whatever else was going on with them, we may never know.
Yui can walk safely in the Mirror World, though she does collapse after a while. She did start to pixelate in the REAL world. More than that. SHE CAN CONTROL the Mirror Monsters, though she seems to have to be in a sort of fugue state to do it. Oh, and there's this reflection of her as a small child which tells her she's going to die on her twentieth birthday. A connection is strongly implied between Yui's condition and the increasing number of Shere Ghosts. It turns out in the series, as opposed to the movie, Yui Kanzaki dropped dead a the age of seven. I mean literally. She fell out of the chair she was in while drawing. No reason was given, and her parents never knew. When Shiro panicked and called for them, they ignored him and left the door locked. They probably thought it was just their whining daughter and snivelling son going on again. Yui's mirror reflection that the pair of them had created came and offered to give her a new life, but explained to Shiro that this life would run out when Yui turned twenty. He accepted her offer, terrified to lose Yui and be stuck alone in that house with no one but their parents. He swore he'd save her.
Thus Yui remembers almost nothing about their parents, who were killed by the smoke when the house caught fight after the first mirror-breach. Shortly after, they were parted. Shiro was determined to save Yui, knowing what would happen to her. In America he became a scientist and worked hard. I don't know why he died, but there it was that happened to him, in America. He opened the Mirror World and it killed him. I suspect that the one we see most of the time is Shiro Kanzaki's Mirror World self.
Yui, once she learned the price of her life, refused it. She could not bear what her brother was doing to himself to save her. She could not bear that her life was being bought at the cost of so many other peoples'. Once she remembered what her true nature was, she took control of it and made herself dissipate. She could still have been saved, but in the end it was Yui's wish that came true. When Shiro would have started the whole thing over again, she imposed her will on it. They broke from the loop. Shiro and Yui Kanzaki died in 1989. They moved into the Mirror World, to live with their adult Mirror World reflections.
Shiro Kanzaki i -- wa -- is... hmm. Well, he's twenty-five years old. Terebikun's Kamen Rider Ryuuki Chozenshu First had this to say about him. "He is the scientific genius who created the Kamen Rider Card Decks and tells the Riders to fight each other. About a year ago (in terms of the story this would have been roughly August, 2001), he disappeared from Seimeiin University after a mysterious experiment. Right after that he began appearing here and there, giving out the decks. He seems to live in some strange, other dimension." They're making an almost laughable attempt to be subtle, here. It should be noted that Reiko knows his name, or rather his name of Shiro Takami. She'd heard about his mysterious disappearance a year ago. Heck, we heard about it in Episode 2. She went to America to research the mystery and found out that he'd died there, before his reappearance in Japan.
He is Yui's beloved older brother. Yui used to be horribly teased by other kids because she talked about seeing monsters inside mirrors, and her big brother protected her from such treatment. Thirteen years ago their parents died in a mysterious fire or some sort of gas explosion. It left the house standing, and the children alive. There were some scenes of them huddled in the smoke-filled house. Yui was taken in by their father's sister, Sanako. But their mother's sister and her husband, Takami, took Shiro, and they lived in America. You can just imagine the enthusiasm with which this was greeted by the children. They had to drag Shiro, practically kicking and screaming, into the taxi. He kept crying that he had to stay and watch out for Yui. He was one big twelve year old.
The last time Yui saw her brother before all this started, he came to her to tell her that, even if he disappeared she should remember he was always watching out for her.
For a long while I thought Shiro Kanzaki was malicious and cruel. He chose madmen to be Riders. He provoked the battles. He murdered Tezuka as surely as if it were his body there instead of Oja's. He controls the monsters. He chooses which ones go to which Riders. Does he really allow them to feed off of people? Eh-heh. I don't know. But I think so. You see, it's all about Yui. It's all about saving her, protecting her. For Yui had died thirteen years ago, and her life-force was replaced by the energy of her Mirror World double. The fire in the house was caused by her stepping into the real world.
Kanzaki is drawn to his Riders. Where they go, he can follow. Once, when Yui took off and he was afraid as he didn't know where, he cut all the Riders off from the Mirror World and then sent Ren and Shinji to track her down. He was not able to get to her until they brought him there. That was interesting. They hear his voice in the depths of their minds. He stirs the violence in their nature, calls them to fight. Shinji resists on a subconscious level, because Kanzaki did not choose him to be a Rider. Tezuka ignored the voice in his mind. But Kanzaki will not be ignored. Time is short, and the Riders must fight each other.
Ayano Sugiyama plays Yui Kanzaki. Kasue Tsunogae plays Sanako Kanzaki. Kenzaburo Kikuchi plays Shiro Kanzaki, who is not Odin. The kids who are Yui and Shiro were played, at least from episode 43 on, by Miho Ozeki (or Kozeki) and Yuuki Hirano. Those other people, I don't have a clue about who they really are.
