Wataru Kurenai is a shy young man who never lies, keeping to himself due to his hygiene obsession. He works at an atelier to continue his father's legacy of violin making with a dream to create the ultimate violin, using strange methods to accomplish it. Wataru is destined from birth to become the current Kamen Rider Kiva as his mother is Maya of the Checkmate Four. He received a button with a diamond encrusted crown on it from her as a baby. He is in fact the younger half-brother of Taiga Nobori. As a boy, Maya left him in the Castle Doran that his father lived in to protect him from retribution from their enemies. However, as the building has since said to be haunted, Wataru was referred to by his neighbors as 'Obaketaro.' He teams up with Kivat to fight the Fangire race and protect humanity from them, only because the Bloody Rose, his violin 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. 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 his power and using it to take him down at the cost of his life.
Wataru's Bloody Rose was created by Otoya and Maya as the 'ultimate violin', later rumored to be a product of Otoya selling his soul to the devil himself, which is not entirely false, in a sense. The Bloody Rose only sounds whenever a Fangire is detected in the area, its music only heard by Kivat and Wataru. When played, the Bloody Rose can also disrupt a Fangire's activity. However, the Bloody Rose is sympathetic to Wataru's mentality as it cracked when he fell into a deep depression. However, after repairing it, Wataru learns the Bloody Rose holds the desires of his father, placing in it his own desire to assume Flight-Style. Over the series, Wataru starts to overcome his fear of the outside world, meeting Mio and falling in love with her while reunited with his childhood friend Taiga. But after gaining the Zanbat Sword, Wataru learns Mio and Taiga are Fangires while they learn his identity as Kiva.
Soon after, Wataru learns that he is Taiga's younger half-brother. Wataru slowly accepts this with intentions of building a world where both Fangires and humans can coexist peacefully. Though hardships from being used by Taiga to attack his friends to Mio's death, Wataru's trip back in time to meet his father opens his eyes. With a stronger resolve, Wataru defeats Taiga to become the new Fangire King as his way of protecting him. After the two finally put their differences aside and defeat the revived Bat Fangire, Wataru and Taiga go into a friendly fight to see who becomes the new King. Days later, just as he is about to play the violin at Nago and Megumi's wedding, a young man wearing a Kivat Belt named Masao Kurenai bursts into the reception claiming to be Wataru's son 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 Masao as Kiva, Raising Ixa, Dark Kiva, and the Arms Monsters at his side, transforms into Emperor Form and confronts this new evil.