Kamen Rider Ryuki (2002-2003)

Thirteen Card Decks were created for thirteen Kamen Riders. They make Contracts with monsters from the mysterious Mirror World, a parallel dimension opposite to our own in which only the Kamen Riders can exist. The Riders draw on their monsters' strength in exchange for feeding them the life force of the creatures they destroy. The creator of the Advent Cards has only one rule: that there can be only one Kamen Rider. The others must be killed, and the sole victor will be granted a single wish, which leads to a conflict known as the Rider War.
All over the city, innocent people are being mysteriously abducted, never to be seen again. During his investigations of these incidents, Shinji Kido – an intern at the online news service ORE Journal – discovers one of the Advent Card decks at an apartment where every reflective surface has been covered by newspaper. He is soon sucked into the Mirror World, discovering the terrifying truth behind the disappearances: people are literally being pulled through mirrors by the monsters of the Mirror World so that they may feed. He is about to be killed by a powerful dragon named Dragredder when he is saved by Kamen Rider Knight: Ren Akiyama. Ren seeks to win the Rider War at all costs. He works with a young woman named Yui Kanzaki, who seeks her missing brother: the master of the Rider War, Shirō Kanzaki. Seeing Ren's strength, Shinji enters the Rider War not for the prize, but so that he may protect innocent people from the threat of the Mirror World, and stop the senseless fighting between the Kamen Riders. With Dragredder as his Contract Monster, he becomes Kamen Rider Ryuki.
He will discover that there are some people who cannot be saved. That you must sometimes fight in order to stop the fighting. The extent to which you must go if you want your deepest wish to come true, and the sacrifices that you must make. In the end, there can be only one Kamen Rider...