After being kidnapped by the cult Gorgom on the night of their 19th birthday, stepbrothers Koutarou Minami and Nobuhiko Akizuki were submitted to a cyborg surgery with the purpose of becoming the candidates for the next Gorgom Creation King. Both of them are called Century Kings and are destined to best each other in a final battle—whoever wins will become the next Creation King. Koutarou escaped before getting brainwashed (which is the final step in the surgery), with help from his foster father, and turned against Gorgom. He soon finds out the horrific truth from his stepfather: Gorgom originally killed his true parents and, since both Koutarou and Nobuhiko were born on the day of a solar eclipse, referred as "Black Sun" and "Shadow Moon." Koutarou, taking on the name of Kamen Rider BLACK, was determined to rescue his stepbrother from Gorgom while protecting Japan. However, later in the series, Nobuhiko emerged as Shadow Moon to fight Koutarou with the survivor becoming the next Creation King.

Formerly henshined into Kamen Rider BLACK. A few years after the events in Kamen Rider Black, Koutarou was captured by the Crisis Empire when the Skull Devils were sent to bring him to them. Refusing to side with the Crisis Empire, they shattered his Kingstone into two pieces, thus rendering him unable to transform into Kamen Rider Black. He was then thrown into outer space, where the sun's radiation mutated his Kingstone and allowing him to turn into a new cyborg, Kamen Rider Black RX (Child of the Sun). Rather than Rider Kicks, Kamen Rider Black RX and his different forms had different finishing moves. Although Black RX and each of his forms were able to execute Rider Kicks, they were not finishing moves. In battle, he wields the Revolcane, a light saber that he uses to impale the monster which ends up slowly sawed in half and then destroyed. Later, he discovers two new cyborg forms, Robo Rider (Prince of Sadness) and Bio Rider (Prince of Anger). The "RX" in Black RX's name represents "RX sunlight," a part of energy in photosynthesis, which his belt is now powered by.

SD Version
The newest Kamen Rider and star of this animated short. BLACK RX is still new to being a Kamen Rider and often leaps before he looks. In the movie, he has fallen in love with an instructor at a local sports club, which Gelshocker kidnaps in a plan to destroy the Kamen Riders once and for all. He has been paired up in a team with Kamen Rider Ichigou and Kamen Rider ZX, the latter of which has taken a liking to teasing the young Rider. His finishing move is slashing his enemies with his Ribolcane sword. He rides in a modified version of the Acrobatter motorcycle.

BLACK RX Does not have a human form in the Kamen Rider SD OVA and Manga series. A flashback indicates that BLACK RX and Shadowmoon have been Kamen Riders ever since they were children, negating the existence of Kamen Rider BLACK.

(Source: Wikipedia)