In the original series, he was portrayed by Hiroshi Fujioka, who also performed most of his own stunts. Later, he was portrayed by Masaya Kikawada in the film Kamen Rider The First and its sequel, Kamen Rider The Next.
The character, before being transformed into a powerful hero, Takeshi Hongo (本郷 猛 ,Hongō Takeshi?) was a college biochemistry student. Born in 15 August 1948, he is an intelligent young man who enjoyed riding motorcycles, in the first episode of the series he witnessed the murder of a scientist at the hands of a Shocker kaijin named "Kumo Otoko" ("Spider Man"). He is then kidnapped and forced to undergo a procedure that turns him into a super-powered cyborg, but escapes before he can be brainwashed to do Shocker's bidding.
After this initial introduction, the series becomes, as with most tokusatsu programmes of the era, fairly formulaic. Each episode sees Hongo meeting another of Shocker's mutants, interacting briefly with supporting characters, such as his mentor Tachibana Tobei, and ultimately defeating the enemy, often with trademark "Rider Kick" or "Rider Punch" attacks.
However, this formula was drastically altered during the filming of episode ten. Fujioka, while attempting to perform a stunt on his motorcycle, shattered his leg. The surrounding episodes were quickly edited or otherwise altered to account for this, but Fujioka's injury forced him to take a longterm hiatus from acting, so production company Toei hired a new actor, Takeshi Sasaki, to play a new Kamen Rider character, dubbed Kamen Rider 2. In the show, it was explained that Shocker had built a duplicate of their original Kamen Rider cyborg in an effort to combat the threat that Hongo posed to their organization. This new character was introduced in episode 13, as Hongo's character was gradually phased out, a process that had been occurring since episode 11.
By the time Fujioka had recovered from his injuries, Kamen Rider had become fairly successful, and Fujioka found other work, so Sasaki continued to portray the main character of the series until episode 49, when Fujioka returned to the series full time. During this time, the two characters worked together as the "Double Riders", and Takeshi's character was written out. By episode 53, through the end of the series with episode 98, Hongo was the sole protagonist, but Sasaki returned for guest spots intermittently, in episodes 72-3, 93-4, and the finale.
Kamen Rider 1 and his partner Kamen Rider 2 next appeared in the sequel Kamen Rider V3. The titular V3 was Kazami Shiro, a junior of Hongo's, and begged them to turn him into a Kamen Rider after the murder of his family by the Destron organisation. Despite initially refusing, when the young man was mortally wounded assisting them, the two Riders transformed him into Kamen Rider V3, possessing both Kamen Rider 1's "skill" and Rider 2's "strength". The Double Riders were apparently killed detonating an atomic bomb, but reappeared alive and well later on in the series.
The Double Riders would make appearances in most succeeding Kamen Rider TV series, with the exception of Kamen Rider Amazon, Kamen Rider Super-1 (although they show up in the Super-1's movie) and Kamen Rider BLACK. Kamen Rider 1 would lead the senior Riders one last time in the final episodes of Kamen Rider BLACK RX. This would prove to be the last time the 10 original Riders appeared together onscreen. Starting with Kamen Rider Kuuga, no past Riders appear in the current TV series from the franchise.