Naoya Kaido, also known as the Snake Orphnoch, is a character introduced as part of Yuji Kiba's renegade group of Orphnochs who seek to coexist with humans rather than exterminate them. His background is marked by a deep personal tragedy: he was once a highly gifted guitarist whose dream of becoming a famous musician was shattered when his own teacher, jealous of his talent, deliberately sabotaged him, causing irreparable damage to the motor skills in his left hand. This loss forced him to abandon music and ultimately led to his death and rebirth as a Snake Orphnoch after being sired by another Orphnoch. Kaido’s personality is eccentric, theatrical, and often loud, frequently masking his deep-seated insecurity and pain with sarcasm and dramatic complaints about his fate. Despite his claims of being a "bad guy," he possesses a fundamentally kind heart and is unable to bring himself to harm innocent humans, which sets him apart from more ruthless Orphnochs. His motivations are driven by a desire to find a place where he belongs, and he follows Kiba largely because the group offers a sense of family and purpose. Within the story, Kaido serves as both a comedic relief character and a reluctant ally to the protagonists, often switching between acting as a villain and a hero. He is fiercely loyal to Kiba and Yuka Osada, though he constantly bickers with Kiba over his idealism and remains oblivious to Yuka’s romantic feelings toward him. His development traces a gradual journey from a cynical, self-pitying outcast to someone who ultimately chooses to protect his humanity, even as an Orphnoch. Key relationships include his contentious but brotherly bond with Kiba, his oblivious companionship with Yuka, and a one-sided crush on Mari Sonoda. He also shares a competitive and often comedic dynamic with Keitaro Kikuchi. In terms of notable abilities, as the Snake Orphnoch, Kaido possesses superhuman strength, speed, and agility, along with snake-like traits that likely include venomous attacks or constriction techniques; his exact power set is not elaborated in great detail, but he is capable of engaging in combat against other Orphnochs and Riders. He also briefly uses a Riotrooper belt, allowing him to transform into a Riotrooper soldier. Throughout the series, he survives the conflicts, and his story ends with him choosing to live as a human despite his Orphnoch nature. In the alternate timeline of the film Kamen Rider 555: Paradise Lost, he and Yuka are killed by the Elasmotherium Orphnoch.