Katai Tayama/Synopsis

History
At least ten years ago, Katai met Doppo Kunikida and became friends with him. Prior to joining the Armed Detective Agency, Katai was constantly hired by various corporations to use his ability as a white hat hacker to steal information on their networks and assess their system countermeasures against potential malicious hackers. Despite the rigor and complex nature of such tasks, Katai completed them at an average rate of mere hours.

A month before he decided to join the Agency, Katai once helped Kunikida with an Agency request from the daughter of a wealthy man to trace the whereabouts of her elusive stalker, a task that proved to be tedious for Kunikida and the police. Within three hours after he agreed to help out, Katai was able to trace the stalker's location, including his residence, workplace, where he was from, and even his bank account.

Eventually, upon Kunikida's invitation, Katai decided to join the Agency, where he laid out his futon and started living. Once, when the Agency had a holiday outing, a new clerk unknowingly locked Katai in his room from the outside. A week later, Kunikida realized the situation and rushed to rescue him, only for Katai to ask why Kunikida went to him. In truth, grasping his situation, Katai got information from the key's manufacturer, got a spare made, and had it sent to a Chinese restaurant to have his meals delivered, living as a shut-in in the office.

With Katai practically living in the Agency office, the Agency members – fearful and amused by this at the same time – depended on Katai when electronics-related problems happened. When such problems were solved, they usually offered him bean-jam buns and tangerines. Katai apparently also became pen pals with the daughter of the wealthy man despite being greatly anxious with conversing with her and other people in general.

The Guild Aftermath Arc
After the Guild war, Kunikida and Atsushi Nakajima pay 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 American organization. After the two arrive at Katai's apartment, Katai is found sulking beneath his futon, at which Kunikida asks him the reason behind his gloom. Katai continues brooding about his misery, claiming that everything besides his futon is hell.

Unable to help Atsushi and Kunikida, Katai breaks the news to them that he cannot use his ability because of a love sickness. He explains how he encountered three days ago the perfect woman of his dreams with such beauty and grace, managing to capture a mere photograph 500 meters away. Despite his emotions, Katai knows that it is a fruitless love, since he cannot talk with a woman save for his futon, Yoshiko. With his help badly needed, Atsushi suggests they find the woman and have Katai confess to her. Kunikida tells Atsushi that doing so might kill Katai, but the latter decides to ask the Agency to help him find the woman.

As they walk down the streets to look for the woman, Katai arrives correcting Ichiyō Higuchi, saying that he is the one confessing, not Atsushi. Moreover, he tells Higuchi that he would never work with a mafioso even after his resignation from the Agency. Atsushi then spots the woman exiting a laundromat, much to the shock of the two. Higuchi decides to chase the woman, but she quickly runs away.

To help the woman cross the busy road, Katai activates his ability on the traffic lights, plunging the vehicles in disarray. Katai and Higuchi remain steadfast to protect and eliminate, respectively, the woman they are after. As the woman reaches a dead-end alley, Katai and Higuchi reach her to see the woman with a ponytail up and putting on her mask, whom Higuchi soon recognizes to be Gin.

Katai founds out that she is a member of the Port Mafia. Nonetheless, Katai hands Gin his confession letter, with his feelings unchanged ever since he first saw her, when he understood the true definition of beauty, even if she belongs in the criminal underworld. Back at his apartment, though initially broken, Katai eventually accepts the rejection.

Cannibalism Arc
Sometime later, Atsushi goes to Katai's apartment to seek his assistance but discovers that the scene and his futon are empty. He realizes 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 and apparently locating their base, when Fyodor Dostoyevsky suddenly comes up behind him and shoots him.

Katai leaves behind a hint of evidence to tell Kunikida that he is safe, planting a key with a secret code only he and Kunikida know that alludes to him being safe. He later shows up with Sōseki Natsume, who reveals having helped Katai and that the latter has discovered the Rats' base.

Katai then helps Atsushi and Ryunosuke Akutagawa find the virus ability user in the enemy's location. After some time, Katai suddenly tells them that he has discovered the virus user's location, two meters behind them.

Decay of Angels Arc
After the Armed Detective Agency is framed as the terrorists, Katai has been confined and being guarded heavily by military police for his ties with the Agency suspects. As he throws a tantrum wishing to at least have his cellphone, a smartphone is tossed at him from the window. Upon accessing it, Katai screams out, and the guards enter his room, shocked to find that he has disappeared.

In truth, Katai, Kunikida, and Akiko Yosano are brought to safety by Ranpo Edogawa by transporting him into a novel Edgar Allan Poe made using his phone. Ranpo then brings Katai, Kunikida, and Yosano into Anne's Chamber, where the other Agency members are waiting.