Kamen Rider Ryuki

The 13th Rider (1x27)

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A young boy learns about the Rider War and steals Shinji's Deck before coming to realize that it is not all it is cracked up to be. Yui, after a confrontation with Shiro in the Kanzaki Household learns a shocking truth and the 4 remaining Riders meet a new Rider, claiming himself to be the "13th".