He teams up with Kivat to fight the Fangire race and protect humanity from them, only because the Bloody Rose orders him to "fight". But in time, Wataru learns of his family legacy and nature as a Fangire, as well as desiring to change into someone just like his father.
He later seemingly matured more by the time Decade's series started as he remained cool and calm throughout the series (although it should be noted that he still seemingly has his kind and caring side as shown in Wizard).
Evidently, Wataru is the one who inherited the mantle of Kiva from his mother, as his full-blooded predecessor was the previous King of the Checkmate Four who attempted to destroy humanity in 1986 with Otoya taking Kivat-Bat the 2nd and using it to take him down at the cost of his life. His name translates to "crimson going over" possibly a reference to his human and Fangire bloodline.
Left by Maya as a child to protect him from the retribution she would face, Wataru lived in the European-style manor that his father lived in. However, as the building has since said to be haunted, Wataru was referred to by his neighbors as Obaketaro (お化け太郎 Obaketarō, literally "Ghost Boy") for his strange appearance, later donning a mask and glasses as an adult to protect him from the "world's allergies" and using a notebook to express himself without talking. His only friend during his childhood was a boy named Taiga, who helped Wataru when he was bullied and mysteriously disappeared.
In 2008, while gathering fish bones in a third attempt to create the ultimate varnish for the ultimate violin like his father, Wataru comes across Megumi Aso, who manages to show him that his "allergies" are all in his head. He later saves her from the Horse Fangire as Kiva, countering her attempt to kill him soon after before leaving her stupefied and later saving her a second time from the Octopus Fangire.
During the events of "Climax Deka", Wataru helped Ryotaro Nogami during his fight against Nega Den-O and the Nega Den-Liner.
When a female lawyer named Aya Natsukawa arrives to Wataru's home with a list of the various people that Otoya had victimized with his antics, Wataru is devastated to know a side of his father his mother never told him. After meeting Keisuke Nago and getting his advice, Wataru performs acts of moral obligation for his father's sins, until he learns that Natsukawa is the Moth Fangire and Otoya's most wounded victim. Unable to fight her, Kivat is forced to summon Garulu to supplant Wataru with his feral rage in Garulu Form so Kiva can kill the Fangire.
When Nago requests that Wataru keep an eye on Megumi in his stead, he meets a young man named Noboru, who is obsessed with Megumi himself. When Noboru reveals himself to be the Sheep Fangire, Wataru learns of Nago's hatred towards Kiva. Saddened but undaunted, Wataru manages to rescue Megumi when she is captured by the Sheep Fangire, destroying him in Basshaa Form. Since the incident, Wataru took to heart what Megumi learned from her mother: the same ideology Otoya follows: "All humans are music". Eventually, Wataru is forced to lose all of Nago's respect for him when he tries to protect the Frog Fangire, Takeo Ōmura. However, Wataru did learn from Ōmura that he needs to make a violin to call his own.
It was later that Wataru meets Kengo, whose rock and roll helps him in becoming more confident in himself. Though Wataru and Kengo have separate dreams, the two promise to help the other out so that both their dreams come true. Wataru ensures his part by protecting Kengo from the Rhinoceros Fangire, accessing Dogga Form. However, Wataru's world is turned upside again when he encounters the Lion Fangire Rook, whom he met when he was suffering amnesia. However by meeting Mami Kurasawa, a former student under his father (of which he is unaware), Wataru learns to cope with the recent Fangire events of who's evil and not by "listening to the voice in his heart."
After begrudgingly following Kengo to a fortune teller to find out who Kiva really is, Wataru loses consciousness. When he awakens, he is more confident in his abilities, but has a very sarcastic attitude. It is revealed that he is possessed, albeit unknowingly, by his father Otoya as a result of the fortune teller's ritual. While using Wataru's body, although unable to maintain possession at times, Otoya teaches Megumi how to overcome her fears of using the Ixa Knuckle and she is able to help Ixa defeat the Ladybug Fangire. When Otoya finally leaves, Wataru has no recollection on what has happened under the influence of his father's spirit.
Soon after this, at a mixer that Kengo set up, Nago has a waitress named Mio fired for accidentally spilling ice cream on his shirt. Feeling sorry for her, Wataru runs after her and finds out that they have a lot in common and they start dating with Megumi's help. However, when a berserk Grizzly Fangire tries to kill Mio, a rage awakens within Wataru that releases Tatsulot from Castle Doran who unleashes Kiva's full power, changing Kiva into his Emperor Form to mortally wound the Fangire. Before Mio took her leave, due to personal issues, Wataru promises Mio that he would get stronger for her. While the Fangire Hunters were being arrested, Wataru remembers Taiga and decides to help them out by him and Kengo joining the Fangire Hunters to battle the Crab Fangire until Nago altered history so that the Cicada Fangire's survival negated the Fangire Hunters' crisis. But in the aftermath of the fight against both Fangires, Nago learns that the button he gave to Maya ended up in Wataru's possession, explaining it to be an heirloom from his mother, setting up the events where Nago learns that Wataru is Kiva.
The fight's aftermath also caused a falling out between Wataru and Kengo when the latter's hand was injured to the point he could never play again and Wataru was trying to keep the truth from him out of not wanting him to worry. Later, Wataru is face to face with a fully grown Taiga, only to be devastated after learning that he and Mio are engaged. This stress, augmented by Kengo's return as the new Ixa user, causes the Bloody Rose to crack. Though he finishes repairing the violin, he realizes something is missing from the Bloody Rose. Then, Jiro appears to him, revealing to Wataru that his father's desires dwell within the violin. When ambushed by a Kukulkan while fighting the Horsefly Fangire, Kivat holds the monster at bay as Wataru plays the Bloody Rose, realizing his father's desires to protect all music. Infusing the violin with his own desire to find his own music, Kiva finally awakens into Flight Style to destroy the Kukulkan. However, this event causes the evoking of a weapon within Castle Doran, the Zanvat Sword of the previous King of the Checkmate Four.
Eventually, Jiro brings Wataru into Castle Doran to pull out the sword. When he discovers that he can't pull it out, he finds Mio running towards him until she is knocked out by the Rat Fangire. After destroying one, he is ambushed by two others before having to fight Kamen Rider Saga. Escaping, he finds Mio, who tells him that she loves him, not Taiga. Wataru becomes happy after this event, until he receives an invitation to Taiga and Mio's wedding. He then appears before Mio and Taiga, defending them from more Rat Fangires, while admitting his feelings to Mio. Found by Jiro, he tries to pull out the blade again, succeeding, though the Zanvat is created to limit the sword's power when it causes his Fangire blood to surface. After a brief fight with Saga, Wataru is exposed by Mio as she is revealed as the Pearlshell Fangire with Saga reverting to Taiga. Wataru runs off in confusion and hysterics before being spirited by Jiro who helps him cope with the revelations. The next day, as he and Mio discuss this, Taiga takes them to meet Maya as Wataru recognizes her. While demanding why she left him to fend for himself, Wataru learns that he is Taiga's younger half-brother.
Although confused as to why Maya never told him about any of this, Wataru slowly accepts his true identity. Wanting to share the truth, he informs Nago that he will tell everyone in the organization that he is Kiva. When Nago arranges a gathering at a restaurant, Wataru also reveals that he is half-Fangire, shocking everyone. Despite this, Shima falsely promises to help him build a world where both Fangires and humans can coexist peacefully.
Later, Wataru is approached by Taiga, who asks him to join the Fangires, and Mio, who asks him to destroy Taiga so he can be the new King. Wataru declines both of these requests, perplexed as to why both sides cannot get along. However, Taiga is persistent in getting his friend to understand his ideology that he asks the Bishop to awaken Wataru's true Fangire powers. This causes Wataru to go berserk and attack his friends, even to the extent of nearly killing Megumi. When he finally returns to normal, he becomes so traumatized from the ordeal that he completely relapses back into a recluse and completely isolates himself within his house, refusing to fight as Kiva anymore.
Though being convinced by Kengo and Nago, it took Maya's intervention to convince Wataru to fight as Kiva again, not as a human or a Fangire but as himself, so that he'll find his own music. Taiga, enraged by Wataru's decision, gives him time to reconsider. When Shima was near certain death, Taiga saved him by transplanting into him a bit of Fangire power. Wataru is appointed as Shima's bodyguard when this is revealed, as Nago was unable to bring himself to kill Shima. Shima then dies in combat with Taiga, enraging Wataru. They battle, and just as Kiva was about to finish Saga off, Mio steps in the way and takes the Emperor Moon Break as she is destroyed. However, finding her after the Bishop fatally wounds her, Wataru is left to think that he killed her and goes into despair with an equally distraught Taiga intent on killing him under the same impression.
But Jiro's interference allows Wataru to go back into 1986, meeting his father face to face just before he dons the mantle of Kiva. Though he originally came to keep his parents apart to negate his existence, Wataru learns that it would be pointless if Mio had never met him and the only way he could honor her is to live a strong life for her sake. He would ultimately play a pivotal role in the death of the original King before returning to 2008 after receiving his father's final words. With a stronger resolve, he charges into battle against the revived Fangires unleashed by the Bishop, saving Shizuka, Megumi, and Nago before confronting Taiga to settle things. In spire of interference, the fight ends with Wataru the winner though he refused to kill Taiga as he take his brother's place as the new King of the Fangires to everyone's shock. Taiga later demands Wataru to fight him again, having claimed to kill their mother to acquire the powers of Dark Kiva.
After their fight is interrupted by the Bishop and his new army of revived Fangires, the two Kivas and the Arms Monsters quickly defeat the Fangires, as the Bishop is destroyed by Nago as Rising Ixa. Wataru reveals that he had taken the mantle of the King for his brother's sake, shielding him from his enemies. By then, the revived Bat Fangire attacks the Kivas, defeating them easily. Wataru is thrown off of a cliff and encounters the spirit of his father near the discarded gauntlet of the original Ixa Prototype, telling Wataru that he lives on inside him and not to lose hope. With his father's encouraging words, both he and Taiga are able to defeat the Bat Fangire with a Snaking Death-Break and Emperor Moon Break combo. When Maya appears, Wataru is put at ease knowing that she was not really killed by Taiga as the two brothers are now on the same page and resume their fight to determine who will be the new King of the Checkmate Four.
Days later, just as he is about to play the violin at Nago and Megumi's wedding, a young man resembling Otoya bursts into the reception claiming to be Wataru's son Masao Kurenai from 22 years in the future. He warns his father of a new threat to humanity and shows him, Taiga, Nago, the Arms Monsters, and the rest of the reception a portal in the sky. In the end, Wataru, with his son as Kiva, Rising Ixa, Dark Kiva, and the Arms Monsters at his side, transforms into Emperor Form and confronts this new evil.
Kamen Rider X
Kamen Rider Amazon
Kamen Rider Super-1
Kamen Rider ZX
Kamen Rider Black/RX
Kamen Rider Kuuga