Kamen Raidâ Ryûki (TV series 2002-2003)

People have begun to disappear in Japan, and nobody seems to care. Cops turn up no results, and the only one investigating is an online newspaper called the ORE Journal. When an overly ambitious intern named Shinji Kidou decides to investigate one of the missing persons cases, he finds himself in an abandoned apartment that once belonged to, what seems like, a seriously deranged man. The windows, mirrors, and any other reflective source in the apartment has been covered up. Young Kidou finds a black deck laying on the floor, and when he later uncovers a window, he is attacked by a dragon. This is only the beginning for Shinji Kidou, who is about to enter the most terrible time of his life. Kidou is helped by Yui Kanzaki, a teenage girl whose brother is among the missing. She tells him of the other world, the mirror world, the world that is only inhabited by monsters, monsters who leave that world for ours when they need to feast. Yui suspects her brother, Shirou, is somehow involved, because he had the knowledge to make the invention of "Card Decks," which allow regular humans to transform into Kamen Riders, and go into the mirror world to murder the monsters. With the black deck Kidou had found in the abandoned apartment, he makes Yui give him the ability to transform into a Kamen Rider, sworn to protect the real world from the horrible wrath of the mirror monsters. What Kidou doesn't know is that there are twelve other Kamen Riders, and they're all not as kind-hearted as he...they are selfish, sadistic, cruel, awful people, who have been given promises for their involvement, for whoever can defeat the monsters, and outlive each other. Nothing will stand in their way, but for brave, enthusiastic, naive Shinji Kidou. As Kamen Rider Ryuki, he is about to enter the mirror world, a world of death and conspiracy, and the odds are not in his favor... - IMDb