If Kamen Rider were to have a dark and older brother, he would be… The Skull Man. While Kamen Rider was in development in 1970 by Shotaro Ishinomori, The Skull Man was born. Too dark and sinister for a kids show, Ishinomori splintered the character off and made him into his own manga, while Kamen Rider went ahead to forge his own path of justice. The Skull Man was later reborn and released in a collaboration between Ishinomori and his protege`, Kazuhiko Shimamoto in 1998, which you can actuall score here in the US, courtesy of TokyoPop.
And now an anime is coming down the pipe. Little is known, but the website has been launched. More as it breaks, so stay tuned to your local Justice Channel.