Kohtaro Minami, alternatively spelt Kotaro Minami is the main protagonist of both Kamen Rider Black and Kamen Rider Black RX. Starting with Kamen Rider Decade and continuing onto most of the later crossover movies, there are two Kohtaro Minamis who are independently Black and RX.
Kohtaro was born on the day of a solar eclipse, along with his stepbrother Nobuhiko Akizuki, within the same hospital. Gorgom noticed this, and both brothers were entitled heirs to their leader, the Creation King's throne. After Kohtaro's parents both died (when Kohtaro was around 3 years old) as a result of an apparent accident, Prof. Akizuki then adopted Kohtaro alongside Nobuhiko as if they were brothers. On the night of their 19th birthday, they were captured by Gorgom and were remade into cyborgs and infused with King Stones. Kohtaro received the "Stone of the Sun" to become Century King Black Sun (世紀王ブラックサン, Seikiō Burakku San). However before the final brainwashing Soichiro Akizuki interrupted the final stage of the process with Kohtaro barely escaping.
Having been pursued by Gorgom after his escape, Kohtaro discovers his newfound ability to turn into the a grasshopper humanoid as a result of their surgery, along with his newfound super strength and cyborg abilities.
After receiving a letter from Soichiro, Kohtaro secretly meets with his foster father and learns the truth about his childhood. Soichiro reveals to Kohtaro that he agreed to join Gorgom in order to receive funding for his archaeological research in exchange for Kohtaro and Nobuhiko to be operated on during their 19th birthdays. When Kohtaro's father had refused to join the organization, Gorgom had him and Kohtaro's mother assassinated. After Prof. Akizuki is killed by five of Gorgom's Spider Mutants, Kohtaro transforms once again and takes the title of "Kamen Rider Black." He then vows vengeance against Gorgom and seeks to rescue Nobuhiko from their clutches. As Black, Kohtaro gains support from Nobuhiko's younger sister, Kyoko, as well as Nobuhiko's girlfriend Katsumi. He would often be found working part-time with the two girls at the Capitola café. He would also gain allies in fight against Gorgom, such as Interpol officer Ryusuke Taki and Gorgom's own Whale Mutant.
Eventually Kohtaro reunites with Nobuhiko; however, Nobuhiko was converted into the perfected Century King, Shadow Moon. In their fight, just when Kohtaro was winning the fight, the Creation King temporarily turned Nobuhiko back into human form. This caused Kohtaro to lose the fight as Nobuhiko turned back into Shadow Moon and launched a surprise attack. In a desperate attempt, Kohtaro tried to convince Shadow Moon that Earth belongs to all living things, and that Gorgom's actions were wrong. Shadow Moon in response stabbed him with the Satan Saber.
Shadow Moon then reached his hand towards Kohtaro's King Stone. Just when it seemed all hope was lost though, Kohtaro suddenly turned back into his human form. The human form of Kohtaro reawakened the lingering humanity within Shadow Moon, causing the latter to retreat.
Despite retaining his King Stone though, Kohtaro's wounds were too grave to heal on his own, causing him to die. In response, his remaining family left Japan and people from across the country lost hope in the battle against Gorgom. Luckily though, the Whale Mutant used a special ritual to resurrect Kohtaro, as well as raising his power. Having been revived, Kohtaro fought his way through hordes of monsters until fighting Shadow Moon and unwillingly defeating him. Kohtaro was then mercilessly taunted by Shadow Moon who told him that he shall feel regret for the rest of his life knowing that he killed his best friend and step-brother Nobuhiko. Despite these cruel words, Kohtaro told Shadow Moon to stay alive so they can later escape together.
Using the Satan Saber wielded by Shadow Moon, Black then fought against the Creation King of Gorgom and destroyed him. The base soon started to self destruct which prevented him from getting Shadow Moon out to safety. Kohtaro then looked at the ruins while saying Nobuhiko's name. With Gorgom defeated, Kohtaro visited the Capitola bar before leaving for unknown parts.
After the defeat of Gorgom, Kohtaro is left homeless until he is taken in by the Sahara family. He became a helicopter pilot for the family's business due to their connections, beginning a new life for himself. Unfortunately, Kohtaro is later abducted into the Crisis Empire's mother-ship. The Crisis Empire was aware of his identity as Black and gave him an offer to join them. Kohtaro refused and fought back but was incapacitated when his transformation ability became disabled. He was ejected into space to float aimlessly until the sun's radiation altered his King Stone. When his pod entered Earth's atmosphere, Kohtaro miraculously survives and immediately discovers he had gained a new power in the shape of an evolved rider form. With his new power, Kohtaro engaged the Crisis Empire forces as Kamen Rider Black RX.
Kohtaro's campaign was long and grueling. In one instance, the family's daughter Hitomi is kidnapped by Maribaron in a plot to make her the Crisis Empire's only living heir. Kohtaro attempts to rescue her using Deathgaron, only to land in the Demon World's Desert of Death. He then ends up in a trap set up by Maribaron. Deathgaron kills Hitomi, but in his grief and sorrow, RX invokes a new Rider power, RoboRider. Using his power, RoboRider fatally wounds Deathgaron before learning that the Hitomi killed was a fake and the real one was being taken to the Valley of Miracles. On his way, he meets Joe the Haze who is under the control of Nexticker, freeing him before they and Shigeru are captured by Maribaron and Princess Karuoriya. It was after they break free that Kohtaro realizes the princess is Hitomi, who was abducted to replace the real princess who died and was force grown by the nearby waterfall. When he attempts to restore Hitomi's memory, Kohtaro is forced to surrender when the Triplon captured Shigeru and Joe. Kohtaro was then placed into a room for a hole designed to counter his abilities as both RX and RoboRider for a slow painful death. His rage to save those he love invokes another new form, Bio Rider. After killing Triplon, with Shigeru's help, Kohtaro succeeded in having Hitomi regain his memory as she and the others are returned to their original ages by the people who originally resided in the valley. With only Joe and the Sahara Children knowing his identity, Kohtaro returns to Earth.
During the middle of the series, Shadow Moon mysteriously returns to seek revenge against him, shortly after, Shadow Moon was finally defeated when RX thrust the Revolcane through Shadow Moon's King Stone, seemingly effectively turning him good shortly before he died after saving two kids. Looking at Nobuhiko's corpse, Kohtaro remembered Shadow Moon's actions and decided that in the end, Shadow Moon truly turned back into Nobuhiko. He then carried the now human form Nobuhiko's body to place in a location where he could rest in peace. In the finale, he was helped by the 10 veteran Kamen Riders to destroy the Crisis Empire.
He will be voiced by Johnny Yong Bosch in the English dub, but reprised by Tetsuo Kurata in the Japanese dub.