After reaching a dead end with the missing Kaoru Kino, Ryo manages to find another Akatsuki attendee named Aki who denied any relation to the Akatsuki. After following her the next day and later saving her from a Jackal Lord off, Ryo attempts to get the truth from her as Shouichi arrives. After waiting for her at her apartment, Aki promises to tell him everything she knew until he collapses in pain. Coming to, he overhears Aki's talking about him to Kaoru, learning that unstable transformation had detrimental effects on Ashihara's body such as cellular decay. Though Aki attempts to help Ryo, offering to introduce him to the other Akatsuki members as a relationship between them blossoms. But when they are attacked by Jackal Lord, Gills holds him off before ending up being the victim of the Agito Capture Plan. He attacks capture team in a fit of rage until Kamen Rider G3 drives him off. Severely wounded, Ryo arrives to Aki's apartment where Shouichi cares for him. By then, Ryo begins hearing voices that urge him to kill. Resisting the voice to the best of his abilities, Ryo realized his feelings for Aki and wanting to be with her to stop the voices.
However, after killing the Jaguar Lord pursing her, Ryo arrives as he sees Kamen Rider Agito with the dead Aki, believing he murdered her and attempts to kill him until G3 arrive to break up the fight. 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 attempting to meet another passenger of the Akatsuki named Masumi; Ryo was attacked by the psychic Sagara and left for dead. However, Ryo was found 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 awaken, escaping with Majima following. Majima was unable to save Ryo on his own until Tetsuya arrived, allowing him to transfer his Agito Seed in Ryo. As a result, Ryo obtain 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 no where 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 new friend dog.
As Gills, the transformation was unstable and carried detrimental effects to Ashihara's body. His body began to degrade from accelerated cellular decay, so severe that he had died from the effects only to be revived thanks to the intervention of others. He was revived once by the Overlord, then again by Mana Kazaya. However, after being resurrected by Mana Kazaya's supernatural healing power, Ashihara discovered that his body chemistry was also altered; able to now accommodate the transformation. Gills can perform his finisher the Gills Heel Claw, when the Gills Heel Claw extended; from that point, he tears away with a flip Rider Kick finisher. His other finisher is Gills Hell Stab, done by throwing the target into the air and piercing their bodies with a stab. Ashihara discovered this evolved form known as Exceed Gills thanks to the sacrifice of Koji Majima. Koji relinquished his Seed of Agito to rejuvenate Ashihara, and activated the Exceed Gills form. Exceed Gills' finisher is the Exceed Heel Claw to restricting his enemy with the Gills Stingers, Exceed Gills performs the Gills Heel Claw attack. Transforming into Exceed Gills also seems to heal Gills of damage he'd suffered, even after once losing his right arm in battle with Formica Regia, his transformation into Exceed Gills caused his arm to regenerate.