Amnesiac young man, who has been adopted the addressed name on a letter found in his possession when he washed ashore. A generous, simple, all-around nice guy, he lives with the Misugi family and does all of the household chores. Using the Altering belt, Shouichi can transform into Kamen Rider Agito, an ancient and powerful warrior and fighter against the Unknown due to being somewhat tied to his forgotten past. As the series progressed, he discovers the reason behind his amnesia and his real name.
His real name's revealed to be "Sawaki Tetsuya." He attempted to contact Tsugami Shouichi, his sister's lover, to find out about her suicide via the letter she wrote for Shouichi. With his search leading him aboard a ferry called the Akatsuki. The El of Water arrived at Akatsuki and assaulted Tetsuya, sensing his churning power, but just as the final blow was about to be dealt, Tetsuya transformed into Agito for the first time. The ensuing battle ended with Tetsuya being tossed overboard, where he was labeled "lost-at-sea" by the authorities. Tetsuya survived and washed ashore but was left with amnesia. He's found by a trio of schoolgirls and brought to the hospital to be cared for by Dr. Azuma Kunieda, and going by the name on the letter in his possession, he assumed the alias of Tsugami Shouichi, who had died at the time, while the real Shouichi was resurrected and assumed the alias of Sawaki Tetsuya, switching their names in the process.
