Katai has black hair and wears small circular glasses and is often found resting in his futon within the comfort of his home. His attire consists of a white button-up shirt, a patterned robe over his shoulders, and dark pants.
He is an anxious, nervous, easily stressed out and unmotivated man, who lies in his futon all day, refusing to go outside or communicate with others. Because of this, Kunikida described him as a hikikomori for reclusively shutting himself at home for extended periods of time, isolating himself from society.
His ability, Futon (蒲団 Futon?), allows him to control any electronics within sight as long as he isn’t touching them. His ability also enables him to process information dozens of times faster than an average human.
Additionally, it seems that Katai can only concentrate wherever his body and mind are most relaxed. Although Kunikida mentioned that he can only activate his ability when he’s wrapped in his futon, Katai has shown to be able to use his ability and change the signal of a pedestrian traffic light even without it.
Katai and Kunikida first met each other 10 years ago. According to Kunikida, when Katai was dragged out of his room to join the Agency, he laid out a futon in his office and began living there and even locked himself in his room to avoid going out.
Kunikida and Atsushi paid a visit to Katai to obtain valuable information regarding the hacking of the Moby Dick's navigation controls during the Armed Detective Agency's climactic battle with The Guild.
Katai is supposedly a remarkable hacker that served as an informant for them. However, he is unable to help the Agency right away due to the fact that he is suffering from love-sickness. Katai had fallen in love with a black-haired maiden and contracted the Agency to help him confess to her as per Atsushi's suggestion.
It is revealed the woman is Gin, and he is consequently turned down. He is saddened by this but claims that it is enough for him. Putting the rejection behind him, he decides to assist in the investigations.
Katai makes a brief cameo when it is revealed that the Agency asked him for help regarding the mastermind.
Sometime later, Atsushi went to Katai's house to seek his assistance but discovered that the scene and his futon are empty. He realized something is amiss, noting that Katai is known to never leave his futon. Kunikida arrives and punches his way through a fake wall to reveal a hidden electronic CCTV recorder that Katai has installed to monitor his room at all times. In the recording, Katai is seen researching the Rats in the House of the Dead, when Fyodor Dostoyevsky suddenly comes up behind him and shoots him.
- His friendship with Kunikida mirrors the real Katai Tayama's friendship with Doppo Kunikida after joining the Literary World (文学界, Bungakukai?) group.