Over the course of the series, he discovers that he is a Singularity Point (特異点, Tokuiten), an individual who can overpower Imagin attempting to possess him, exist outside of time, and is largely immune to timeline changes.
Later in the series, he acquires the K-Taros (ケータロス, Kētarosu) cellphone, which allows him to fuse with all of his Imagin partners at once to assume Climax Form (クライマックスフォーム, Kuraimakkusu Fōmu), and the DenKamen Sword (デンカメンソード, Denkamen Sōdo), which he can combine with a Rider Pass to assume Liner Form (ライナーフォーム, Rainā Fōmu) and use to channel his Imagin partners' powers.
While transformed, he wields the Ghost Imagin's sword, which allows him to perform the Terminate Flash (ターミネイトフラッシュ, Tāmineito Furasshu) Full Charge.
"Kotaro" (小太郎, Kotarō), as nicknamed by Hana, is Ryotaro's 11-year-old self who appears exclusively in the film Kamen Rider Den-O: I'm Born!.
Hana (ハナ) is a Singularity Point, passenger of the DenLiner, daughter of Airi and Yuto, and niece of Ryotaro who originates from a future that the Imagin destroyed.
In pursuit of this goal, she convinces Ryotaro to become Kamen Rider Den-O and often serves as an enforcer to keep his Imagin partners in line, though she eventually comes to see them in a better light.
Urataros (ウラタロス, Uratarosu) is a suave, charming yet deceitful sea turtle/Urashima Tarō-themed Imagin who encounters Ryotaro after possessing him to use his status as a Singularity Point to pursue personal freedoms.
Ryutaros (リュウタロス, Ryūtarosu) is a childish dragon/Taro the Dragon Boy-themed Imagin and a former member of Kai's gang who displays a love of breakdancing and animals.
Yuto Sakurai (桜井 侑斗, Sakurai Yūto) is a spoiled, immature, and wrathful former high school student who was recruited by his future self to become Kamen Rider Zeronos (仮面ライダーゼロノス, Kamen Raidā Zeronosu) and utilize the ZeroLiner (ゼロライナー, Zerorainā) steam locomotive to protect the timeline from rogue Imagin, though memories of him across the timeline are erased every time he uses his Rider powers.
Taking his mission seriously, Yuto abuses his Imagin partner Deneb and initially sees Ryotaro as being unfit to be a Kamen Rider because of the latter's desire to protect people and for refusing to use dishonest methods.
While transformed, he wields the ZeroGasher (ゼロガッシャー, Zerogasshā), which has a Saber Mode (サーベルモード, Sāberu Mōdo) for performing the Splendid End (スプレンデッドエンド, Supurendeddo Endo) Full Charge and a Bowgun Mode (ボウガンモード, Bōgan Mōdo) for performing the Grand Strike (グランドストライク, Gurando Sutoraiku) Full Charge.
"Yu" (ユウ, Yū) is the nickname of Yuto's 11-year-old self who appears exclusively in the film Cho Kamen Rider Den-O & Decade Neo Generations: The Onigashima Warship.
Having lost his mother to an unspecified disease and his father going away for a business trip, Yu is forced to move in with his grandmother in the countryside, where he has trouble making friends.
Deneb (デネブ, Denebu) is a polite and loyal yet over-apologetic crow/Benkei-themed Imagin partnered with Yuto who serves as his butler, parental figure, and combat aide.
She was originally meant to marry Yuto prior to the series, having becoming pregnant with Hana, but Sakurai was forced to erase himself and their child from Airi's memories to protect her from Kai.
Naomi (ナオミ) is the hyperactive and perky yet level-headed waitress of the DenLiner's dining car who, despite her best efforts, often makes coffee that tastes terrible to humans, but is irresistible to Imagin.
During the events of the film Kamen Rider Den-O & Kiva: Climax Deka, the Owner establishes the DenLiner Police, names himself its Chief Inspector, and takes up a DenGasher modified to shoot tranquilizer flags.
The "Past Man" (過去の男, Kako no Otoko) is Yuto's mysterious future self and Hana's father who investigates the Imagin's activities.
He was originally set to marry Airi until Kai's plans forced him to erase himself from the timeline to protect her and Hana, go into hiding to stop him, and recruit his younger self to assist further.
Six months prior to the latter becoming Kamen Rider Den-O, Sieg intended to form a contract with a pregnant woman named Shiori Takayama, but was trapped within a deep sleep by her baby, Yusuke.
[1][2][3] Additionally, he can fuse with Ryotaro and the Taros Imagin to transform into the winged Kamen Rider Den-O Super Climax Form (超・クライマックスフォー, Chō Kuraimakkusu Fōmu).
Gaoh (牙王, Gaō, Fang King) is a thief-king who became disillusioned with the idea of time and seeks to take over the space-time continuum via the power of the GaohLiner (ガオウライナー, Gaōrainā), a legendary crocodile-themed time train formerly known as God's Train (神の列車, Kami no Ressha) that was created by an ancient civilization to erase Singularity Points before it was sealed for fear of its power.
While transformed, he wields the serrated GaohGasher (ガオウガッシャー, Gaōgasshā) sword, which allows him to perform the Tyrant Crash (タイラントクラッシュ, Tairanto Kurasshu) Full Charge.
He eventually discovers Hana is his target, but is ultimately killed by Kamen Rider Den-O and his allies, causing him to dissolve into sand and take several Imagin with him.
During the events of the film Cho Kamen Rider Den-O & Decade Neo Generations: The Onigashima Warship, Kotaro gains access to his own version of Vega Form while possessed by Deneb.
Neotaros (ネオタロス, Neotarosu), also known as Teddy (テディ, Tedi), is an Oni/Momotaros-themed Imagin and Kotaro's partner who can transform into the Macheteddy (マチェーテディ, Machētedi) sword/rifle hybrid for his use in combat, which allows him to perform the Counter Slash (カウンタースラッシュ, Kauntā Surasshu) Full Charge, and first appears in the film Saraba Kamen Rider Den-O: Final Countdown.
While transformed, he wields the SavageGasher (サヴェジガッシャー, Savejigasshā) sword, which allows him to perform his own version of the Terminate Flash, along with a whip and exploding tops.
After being abandoned by his mother and Daiki Kaito steals a Kurosaki family heirloom, the combination of these tragedies led him to distrust humans and rely solely on Eve as both his moral support and mission partner.
Lacking a corporeal form, Eve resides in the Rider Pass and G Den-O's dobermann-like breastplate to supply Reiji with his abilities of time travel, near-future prediction and compensating for his blind spots in combat.
[8] Kajiwara Kagetoki (梶原 景時) is a fictionalized version of the historical figure of the same name and a Time Jacker who appears exclusively in the novel Kamen Rider Den-O: Deneb Kanjinchou.