Ryotaro Nogami (野上 良太郎, Nogami Ryōtarō?) is a young man with an intense streak of bad luck. Having lost his parents at a young age, he lived with his grandmother and older sister Airi, visiting his parents' library/café called the Milk Dipper as often as possible. He also kept a photograph of his parents until he lost it, regretting that day ever since. He and Airi eventually reopen the Milk Dipper along with Airi's fiance. However, her fiance mysteriously disappears a month before their wedding and she is suddenly struck with amnesia of all events relating to him. Ryotaro begins to take care of Airi and help at the Milk Dipper. He makes sure that she is always happy and never reminded of her fiance and become burdened by the loss, even though he questions if it is right for Airi to forget completely. Ryotaro will help anyone in need if he has the ability to do so, regardless of his bad luck; he also refuses to accept self-sacrifice as the only means to protect others.
One day early in 2007, Ryotaro comes across the Rider Pass, the DenLiner, Hana, and becomes possessed by the Imagin Momotaros. Hana discovers that Ryotaro is a Singularity Point, a human immune to changes in history as well as the lure of the Imagin's wish granting. This traps Momotaros in Ryotaro's body and enables both to become Kamen Rider Den-O (Sword Form). As such, Ryotaro is drafted by the Owner of the DenLiner to become a keeper of space-time; Kamen Rider Den-O destroys Imagin that travel to the past to change the future, enabling that the future of the humans remains intact. Along the way, Ryotaro becomes contracted to the Imagin Urataros, Kintaros, and Ryutaros (collectively called the Tarōs), enabling Den-O to transform into additional forms.
Ryotaro also discovers the existence of Yuto Sakurai, the younger incarnation of his sister's fiance, and his Imagin partner Deneb who become Kamen Rider Zeronos. The group also help the Imagin Sieg who nearly ceases to exist, and later use his partnership with Ryotaro to fight the evil Kamen Rider Gaoh when the Edo period is threatened (the events of Kamen Rider Den-O: I'm Born!). It is also in this travel back in time that Ryotaro meets his younger self dubbed "Kotaro" who becomes Kamen Rider Mini Den-O to help with the fight. When three of the Tarōs' existence is threatened by their part in defeating Gaoh, Ryotaro's feelings for the friendship he has with them creates the K-Taros cellphone, bringing them back to life and allowing him to become Climax Form.
When Kai, the Singularity Point of the Imagin's future, appears to further his plan to destroy the Junction Point to ensure that the flow of time ensures that the Imagin will exist rather than humans, Ryotaro's sync with the Tarōs begins to fade, forcing him to fight on his own as Liner Form. As he eventually discovers that the Junction Point is the unborn child of Airi and Sakurai, Ryotaro discovers that they had sacrificed their love and marriage for the sake of their child through Airi's ownership of the ZeroLiner and that their child is none other than Ryotaro's travelling companion Hana (who has been de-aged and become known as Kohana). Ryotaro, the Tarōs, Yuto, and Deneb work together to save Hana and defeat Kai, ensuring that the Imagin will never exist. As Ryotaro begins to lament that he will never see the Tarōs again, they (along with Sieg and Deneb) make themselves known, having been saved due to their time with the Kamen Riders. The Owner tells Ryotaro that his work is done, and Ryotaro gladly relinquishes the Rider Pass, knowing that he will meet the Tarōs in the future.
Some time after this, Ryotaro is asked by the Owner to help in finding other Imagin who survived Kai's defeat and disappearance from history. It is during this time Ryotaro comes across both the evil Negataros and the half-Fangire Wataru Kurenai (the events of Kamen Rider Den-O & Kiva: Climax Deka). After Negataros is defeated, Ryotaro is allowed to live his life as he wishes once more. However, he is abducted by Shiro and contracted to the Ghost Imagin, enabling him to briefly become Kamen Rider Yuuki Skull Form. When the Tarōs and his grandson from the future Kotaro save him, he becomes Den-O once more to defeat Shiro and save all space-time from being reversed such that everyone alive becomes dead and all those who have died come back to life, as well as ensuring that his ancestors in this time period meet (the events of Saraba Kamen Rider Den-O: Final Countdown).
Later, Ryotaro becomes de-aged in a similar way to his niece Hana, and must assist his grandson and Tsukasa Kadoya save space-time from the Oni Brothers Goludora and Shilubara (the events of Cho Kamen Rider Den-O & Decade Neo Generations: The Onigashima Warship). While still in his younger incarnation, Ryotaro also helps Yuto save Airi from the Piggies Imagin, help Kotaro and a young woman deal with her contract with the Mantis Imagin, and inadvertently assist Daiki Kaito steal an item from history and escape from the Time Police (the events of Kamen Rider × Kamen Rider × Kamen Rider The Movie: Cho-Den-O Trilogy).
Throughout the original television series and the first three films, Takeru Satoh portrays the character of Ryotaro Nogami. Beginning with the character's brief appearance in Kamen Rider Decade, and continuing throughout the Cho-Den-O Series, Takuya Mizoguchi portrays Ryotaro. Mizoguchi had originally portrayed the younger iteration of Ryotaro called "Kotaro" in the Kamen Rider Den-O: I'm Born! film.
Read more about this topic: Kamen Rider Den-O (character)