|Shouichi Tsugami is a young man with no memory of his past and is living with the Misugi family. A generous, simple, all around nice guy, Shouichi's ideal life is to just work in his garden and please his foster family. He had a funny habit by addressing someone that he didn't know as Koungouji-san. He fights as Agito due to the Unknown being somewhat tied to his forgotten past. His real name was Tetsuya Sawaki, the younger brother of Yukina Sawaki who had a relationship with a man initially named Shouichi Tsugami. Yukina and the real Tsugami participated in a research study conducted by Prof. Nobuyuki Kazaya (Mana's father) on human paranormal abilities. Yukina, possessing such capabilities, was chosen as a subject. As her powers matured (showing evidence of evolution into the first Agito), she committed suicide. Hearing of his sister's suicide, Tetsuya attempted to contact Tsugami, with his search leading him aboard the ferry Akatsuki. Unaware that the real Tsugami himself had committed suicide as well, Tetsuya searched for him without luck. Eventually, he discovered the lifeless body of a young man who reveals himself as the Overlord of Light, awakening Tetsuya's powers to become the second Agito before vanishing just as the El of Water arrives.
The El assaults Tetsuya until he transformed into Agito for the first time. Tetsuya was tossed overboard and washed ashore, left with amnesia. He was brought to the hospital to be cared for by Dr. Higashi Kunieda, he assumed the name Shouchi Tsugami from the addressee in the envelope he carries. Shouichi would be placed under Dr. Kunieda's care for some time until he was transferred to Professor Misugi. There he would live a quiet, somewhat normal life, with no cares of whether he'll remember his past or not. However, the presence of the Jaguar Lord Panthras Letus re-awakened his power as Agito, albeit involuntary and feral (to the point of attacking an underdeveloped Kamen Rider G3, Makoto Hikawa, almost fatally). This left him confounded until Mana Kazaya talked him into using his powers to protect the people he cares for, with her the only one in the Misugi household aware of it. By accident, Shouichi cross paths with Tomoko Miura, one of the survivors of the Akatsuki cruise and offered to help him recover his memories. However, with Tomoko being murdered, Shouichi never met her, resulting in his unjust arrest for her murder until he was eventually cleared by Mana through pointing Hikawa to the actual culprit, the Overlord of Darkness.
Shouichi finds Taichi dreadfully ill. But as he and Mana care for him, Shouichi meets a young woman named Aki Sakaki who claims to be hired by Professor Misugi as a housekeeper. But during the incident with the Jackal Lord, Aki claims herself to be Shouichi's high school sweetheart. Shouichi then moves in with Aki to regain his past, taking a job at the Hanamura Bakery. Before the owner dies, he recieves recipes and takes over the bakery with Mana's aid. One day sensing the Jaguar Lords, he finds Aki running off. After killing her pursuer Panthras Cyanus, Agito is overpowered by the Jaguar Queen Panthras Magistra and Panthras Rubeo. The next day, he was formally introduced to Ryo Ashihara and learned Aki is his careaker. Soon after, Aki apologized to him for using him while warning him that it's better if he never remembers. Later, sensing an Unknown attack, Shouichi finds Aki having her neck snapped by Magistra. Though he kills Aki's murderer, a heartbroken Ashihara, assuming the form of Kamen Rider Gills, attacks him on the assumption he murdered her. As a result, by chance, Shouichi momentarily recovers his memory after his battle with Gills, allowing access to his Trinity Form but eventually relapsing into amnesia again.
After countless battles, Shouichi faced the El of the Water again. Anxiety and fear of confronting his 'killer' ultimately broke his fighting spirit. But with the help of Mana, Shouichi rekindled his bravery and assumed the ferocious Burning Form to defeat the El. When his old caretaker, Dr. Kunieda, came to pay a visit Shouichi transformed into Agito Burning Form, losing his composure in a fit of rage, and inadvertently attacks Dr. Kunieda. The shame of the accident weighed heavy on him, falling into self-loathing. But thanks to Dr. Kunieda's reminder to value of his life, he was achieved the Shining Form and defeated the Beetle Lord. This new conviction and form would prove essential when the El of the Water returns, eventually disposing of him once and for all. After recovering from amnesia, Shouichi is drawn into the murder of Nobuyuki Kazaya, Mana's father and discovered his study of paranormal psychic ability and his test on her dead sister. The huge emotional and psychological shock cast Shouichi into fierce anxiety and depression, ultimately choosing to relinquish his power to the Overlord of Darkness. He soon realized this grave mistake when the Lords targeted Mana, almost getting killed as well if not for the intervention of Another Agito, Kaoru Kino. Choosing to confront the Overlord of Darkness together with him and Ryo, Mana encourages Shouichi to once again fight as Agito, actually landing a punch on him through Hikawa's aid.
Shouichi, along with Ryo and Kino, regains their respective powers, defeating the Hedgehog Lord and forced the Overlord to retreat. When supernatural murders ceased, Shouichi reconnects with his old cooking teacher, Kuramoto, and accepts a job as chef in his restaurant and meets Kana, a distant reclusive young lady driven to succeed as a chef in honor of her father. Kana, however, is experiencing evolution into an Agito herself. Unable to comprehend the change, she tries to commit suicide, only for Shouichi to futilely attempt to save her if not for the timely appearance of a man named Tetsuya Sawaki, who saw it as his chance to redeem himself for not saving Yukina. Immediately attacked by the El of Earth, Shouichi chose to transform in front of Kana, revealing himself as also an Agito. After a scuffle forcing him to use Shining Form, the El retreats only to be pursued until Shoichi sees the Overlord preparing the genocide of humanity. Revitalized by the Overlord, a strengthened El of Earth and El of Wind fend off Shouichi until his fellow Riders Hikawa and Ryo arrive to aid him. Together, they defeated the Els and the Overlord himself falling to Agito's Shining Rider Kick.