Ryo Ashihara (葦原 涼 Ashihara Ryō) is Kamen Rider Gills (仮面ライダーギルス Kamen Raidā Girusu, Masked Rider Gills), an anti-hero (later, protagonist) character of the 2001 Kamen Rider Series installment, Kamen Rider Agito.
He was formerly a star swimmer from Jyohoku University, but Ashihara's future as a swimmer came to an end after a near-fatal car accident. That event sparked Ashihara's metamorphosis into Kamen Rider Gills, a flawed form of Agito. The loneliness from being rejected and feared by those he knew fueled his lust for battle, which in turn made Gills increasingly wild. His ferociousness is so great, that Gills appears as brutal in the eyes of those who witness him in battle; wildly brandishing his claws and tendrils at his adversaries.
After the car accident, Ryo began to mysteriously undergo a metamorphosis. However, this came at cost of all those closest to him, leaving a signature of resignation to his swimming coach, who began to avoid him out of fear when he demanded to know the nature of leaving the University. Ryo turned to his former girlfriend Mayumi Kitahira for help after avoiding her for three months. But soon enough during the Snake Lords' attempt on Mayumi's life, Ryo underwent the partial evolution into an imperfect form of Agito. This forces him to stay away from Mayumi, who was frightened by his appearance to the point of moving out of town. But because he was an imperfect copy of Agito, Ryo's lack of a stabilizer made his transformation unstable, carrying with it detrimental effects on Ashihara’s body such as cellular decay.
The OverLord found Ryo, prolonging his life for a while longer. After attacking the OverLord out of fear, Ryo learns that his father's body had been found, believed to have died of a nervous breakdown. To learn the truth behind his changes and his father's suicide, Ryo sought answers from the other survivors of the Akatsuki, using his father's black book to find them, starting with Saeko Shinohara, protecting her from the Zebra Lords. During his attempts to kill the Lords, he fell in love with a survivor, Aki, who was killed by the Lords, though by the time Kamen Rider Agito arrived, Ryo came to believe that Agito was her murderer. But furthermore, many of the surviving Akatsuki members came to see Gills as the one out to kill them.
Ryo befriended a boy named Kazuki, whose parents were killed by the Sea Urchin Lord and wanted to evade the truth of what had happened. After seeing Ryo fight as Gills, the boy came to terms with the truth and had Ryo drop him at church to pay his final respects. But while attempting to meet another passenger of the Akatsuki named Masumi; Ryo was attacked by the psychic Sagara and left for dead. However, Ryo was founded by Tetsuya Sawaki, who asked the OverLord to resurrect him like in the past.
But when the OverLord refused, in fear of shortening his life more, Tetsuya attempted to have the newly-awakened Mana Kazaya resurrect him instead. Because of Mana, Ashihara discovered that his body chemistry was also altered; able to now accommodate his transformation. After fighting the Orca Lord, Ryo was found by Tetsuya, who reveals the Lords' fear and Ryo's genetic relation to Agito. Furthermore, he learned of Shouichi's identity as Agito and the truth behind Aki's death.
Eventually, Ryo was forced to fight Kaoru Kino, outmatched by the man's Agito form and forced to run upon being wounded. But Ryo is mysteriously suffering chest pain that would kill a normal person. Taken to the hospital, Kino attempted to use the operation as an excuse to finish the job. But at the last second, Kino lost control over his arm as Ryo awakened, escaping with Majima following. Majima was unable to save Ryo on his own until Tetsuya arrived, allowing him to transfer his Agito Seed to Ryo. As a result, Ryo obtains the ability to assume Exceed Form. He used his new power to defeat Kino, diving after him. It was that act that reminded Kino of his intent to save lives, leading to his eventual sacrifice.
After Kino's death and with the Lords nowhere to be found, Ryo befriended Risa Mishihara. However, he took her OverLord-induced death hard and attacked the El of Ground, only to be defeated and left for dead. However, Gills arrived to aid Agito and G3-X to finish their fight with the Lords for good. In the epilogue, Ryo left town and is wandering the world with a young puppy.
Kamen Rider X
Kamen Rider Amazon
Kamen Rider Super-1
Kamen Rider ZX
Kamen Rider Black/RX
Kamen Rider Kuuga