Seto's parents have been out of the picture since he was younger. Because of that, his small body, and his timid nature, he always had trouble speaking with people and as a result, was bullied. Because he had no people friends, he considered an abandoned dog to be his only friend, and wished that he could "understand how someone (or something) felt without using words."
On one stormy day, he witnesses some kids trying to throw his dog friend into a river. Despite his efforts, the dog was thrown in, and Seto also threw himself into the river in order to save it. He came into contact with the Kagerou Daze and was possessed by the snake with the "Stealing Eyes" ability, acquiring the ability to "hear" information regarding those he lands his eyes on.
He eventually was adopted by the Tateyama family together with Kido and Kano. Together they created the Mekakushi Dan after being encouraged by Ayano.
One day he wandered into the forest and found Marry's cottage after hearing her thoughts[1]. After knocking at the door and receiving no answer, he entered it on his own, only finding Marry cowering on the ground. At first, she was afraid of him, believing that she would turn him to stone if he looked into her eyes. Seto misunderstood her mumbling to warn him, thinking she was just shy and walked over to her to comfort her by showing her his music. As she had learned to control her power over the years, he was not affected, and they became friends. Marry reminded him of his past, weak self, so he wanted to become stronger and show her that she can be strong as well[2]. He continued to visit her after that meeting. An unknown amount of time later, he took Kido with him to Marry's cottage, and she joined Mekakushi Dan.[3]


Seto is speaking to a bird from a window as Kido calls him to prepare breakfast. After they then check up on Shintaro, who woke up after the ordeal with the terrorists in the department building, he stays in the room while the others introduce themselves as Mekakushi Dan. In the anime, he was caught by the terrorist along with Kano and Shintaro.