|This article is about a/an character in Kamen Rider Ryuki.|
Yui Kanzaki (神崎 優衣 Kanzaki Yui) is a main character in Kamen Rider Ryuki. She is the sister of Shiro Kanzaki, the creator of the Decks, and she, with Shiro, is the partial creator of the Mirror World and its Mirror Monsters. She spent most of the series opposing her brother on the Rider War.
Yui at heart is a young innocent girl who is caring to anyone she meets. However, this trait is what causes her the most trouble as a majority of the people around her are associates of the Rider War and only wish to use her to their advantage. This is shown is Episode 6, when Masashi seeks her out.
Yui originally lived with her grandmother, but when her grandmother went overseas, she met Ren somehow, although how this occurred is not specified in the series. Coincidentally, Ren was a Kamen Rider who had received his deck from Shiro at some point in time in Japan. Yui became somewhat of Ren's accomplice and would accompany him to fights where he would enter Mirror World and fight.
Time passed and eventually she also met Shinji, who would became Kamen Rider Ryuki. Her grandmother returned from her overseas trip and she began to help out at the cafe that her grandmother owned. She was kidnapped by Kamen Rider Scissors to use as bait to lure out Shiro but Ren was able to rescue her before that happened. Later, she went to Seimeiin University to inquire about her brother but everyone she asks was much less than helpful. After finding her brother in a yearbook, Her past; which she previously had no memory of, began to reveal itself to her and she met Professor Ejima who worked alongside Shiro during the development of the decks. Ejima died in her arms but not before he told her that everything would have been fine if she had never existed.
She gets a part-time job at ORE Journal while the Chief-Editor recovers.
After continually bothering Nakamura about her brother, he eventually gives in and provides her with a number of papers about Shiro. She learns about a woman named Eri (who coincidentally again is Ren's fiancée) who is in a coma at the hospital.
She is eventually kidnapped by Jun who uses her as bait to lure out Shinji and Ren into battle. Eventually, she manages to find a picture of her old house and goes with Tezuka to have a look but can't bring herself to go in.
Throughout the series, her past continues to unravel even further, she learns many things about her childhood, including that the drawings she and Shiro had created as children were the basis for the Mirror World's monsters. She and Shiro had had neglectful parents who kept them locked up in the attic for long periods of time, and when Yui fell ill, she and Shiro could not get their parents' attention. When she, the original Yui, died, the Yui of the Mirror World (as Shiro and Yui had also drawn themselves) asked to inhabit her body so she could live in the real world. Shiro, wanting his sister back by any means, allowed it. The Yui of the series is this mirror version, and had forgotten.
The Mirror Yui had told Shiro that on her twentieth birthday, she would cease to exist along with the original Yui's body. The purpose of the Rider War was to prevent this from happening.
Yui begins to fade, just as predicted, as she nears her 20th birthday. Even though Shinji, Ren, and Shiro do all that they can to save her, she fades away even though the deadline had not quite been reached. She later appears to Shiro, appealing him to stop his destructive course of actions, saying that to be saved in that manner was not what she wanted, and that she would rather make the mirror world a happy place that would not cause suffering. Shiro agreed, and there is a scene in which she and Shiro, alongside their child selves, draw together, but this time, with happy scenes instead of monsters.
The scene makes no literal sense, but it is believed that Shiro used the power behind Time Vent (even though Odin was fighting Ren at the time) in order to alter history so that the Mirror World was never dangerous and the Rider War never happened. In the new timeline that results from this, there are no Mirror Monsters or Kamen Riders, and all those harmed by the Mirror World are seen to be living normal lives. However, when Ren and Shinji meet the Kanzaki's grandmother, she is alone and the only picture of Yui is as a child. As such, it would seem that in the new timeline, Yui's death as a child was not prevented, and Mirror Yui never took her place.