Shinichi Tsukumoya

Shinichi Tsukumoya (九十九屋 真一, Tsukumoya Shinichi) is the narrator of the light novels, an author, and an online information broker of sorts. Claiming to be in his chatroom 24/7, Shinichi acts as the prime Information supplier for Izaya Orihara. In order to contact Shinichi, Izaya often logs into his chatroom.

Shinichi claims that he usually knows what his clients are going to ask because he can see what they were looking up on their computers prior to their visit. Although he says this, Shinichi claims that he is not hacking anything.

Background
Shinichi and Izaya Orihara go back several years. The online informant first contacted Izaya when Izaya was still in high school. Izaya had created a game in his free time, although he had only given a copy of the game to Shinra Kishitani, Shinichi somehow got his hands on it. After sending Izaya a taunting email about the game, their interactions begin.

It is implied that Shinichi does not exist in the real world but only on the internet as a sort of AI.

A character in Vamp!, another work in the Naritaverse, is called QAWSED "Hackey Mouse" and claims to only exist on the web as part of a race of beings who live in digital form. It is likely, though not outright stated, that Shinichi is a member of this same race.

He serves as one of Izaya's primary information suppliers and one of the only people who can genuinely get Izaya upset just by talking to him. This is primarily due to the fact that Shinichi always seems to have the upper hand on Izaya when it comes to gathering information.

When Ruri is looking for information about her father after his disappearance, Shinichi lets her know he is already dead, this sparks her desire for vengeance and her career as the serial killer Hollywood.

Hollywood Arc
Shinichi introduces himself by claiming that he watches all the events that take place in Ikebukuro while having only a small part to play in some of them through the influence of the internet. Later, while talking to Izaya in their private chat room, Shinichi teases Izaya for being left out of all the excitement in Ikebukuro. He informs Izaya of the now-missing Jinnai Yodogiri's connection to the Awakusu-Kai, the identity of the serial killer Hollywood, and that Egor's connection to Celty was the result of the actions of his own sisters. Shinichi further teases Izaya about Shizuo before Izaya threatens him and signs off.

Adabashi Arc
Shinichi gives Mikado information on who is misusing the Dollars name when Mikado decides to start 'cleaning up' the Dollars. He later provides free information for Ruri Hijiribe when she attempts to search for information on her father's residence following his death.

Final Curtain Arc
Shingen Kishitani makes a Twittia account name 'Kitshi' where he rambles about all of Nebula's secrets and how it wouldn't matter since no one would hear him. There, Shingen considers the possibility of immortals such as vampires and how amazing this day and age has come where the sea of information connects humans like synapses, like how brain cells connect. Then Shingen considers the possibility of how there could be a superior existence that are like all of humanity's brain put together and Shinichi interrupts him by saying that such existence could have already been born. He adds how he can not perform any action outside of the Internet and that he is not a 'deus ex machina.' Shinichi also warns Shingen about Seitarou Yagiri making a move to capture Namie Yagiri.

After the the final events of the story, Shinichi shuts down the Dollars site completely.

Trivia

 * Walker mentions Shinichi as a 'professional' since he has published many books.
 * He is never directly mentioned in the Anime, though he has a few unnamed mentions.
 * One of the titles of the books Shinichi has published is titled Ikebukuro Counterattack (also translated as Ikebukuro Strikes Back) published by Media Wax, Japan. There are at least three volumes of this series.
 * Kasane Kujiragi considered Shinichi to be the second biggest obstacle next to Awakusu-kai being the first.
 * An excerpt from the afterword of the first volume of this series is used as an introduction to volume four of the light novels, and an excerpt from the afterword of Ikebukuro Strikes Back II is used as an outtro to the same novel.
 * Shinichi introduces himself in an excerpt from the forward of Ikebukuro Strikes Back II, inserted into the middle of the fourth light novel.