Shinichi Tsukumoya

Shinichi Tsukumoya (九十九屋 真一, Tsukumoya Shinichi) is the narrator of the light novels, an author, and Izaya Orihara's primary information supplier. He is usually contacted ia his chatroom, where he asserts he is online 24/7.

Shinichi claims that he usually knows his clients' concerns because he checks their personal computers' browsing history in advance of their visits. However, he insists that what he does is not hacking.

Background
During Izaya Orihara's high school years, Shinichi sends him a taunting email about a game Izaya had made in his spare time. As Izaya had only sent Shinra Kishitani a copy of his game, it is clear that Shinichi acquired it without his consent or knowledge. Having established both his skill and the upper hand from the moment of first contact, Shinichi sets the stage for years to come.

Shinichi currently serves as one of Izaya's primary information suppliers and is one of the only people who can genuinely upset or irritate Izaya simply by talking to him. This is primarily due to Shinichi's continued apparent superiority when it comes to gathering information, as Izaya is often one step behind him.

Shinichi is implied to exist solely on the internet as some sort of AI, which would explain his ubiquitous presence and information access. It is likely, though not yet explicitly confirmed, that he is a vampire in the mode of QAWSED, a Vamp! character whose soul is said to be fused with the digital plane.

Hollywood Arc
Shinichi introduces himself as someone who watches all that occurs in Ikebukuro, and as someone who occasionally influences certain events by way of the internet.

Later, Shinichi teases Izaya in their private chatroom over all the excitement in Ikebukuro Izaya is missing out on. He informs Izaya of the now-missing Jinnai Yodogiri's connection to the Awakusu-Kai, the identity of the serial killer Hollywood, and that Izaya's sisters are in part responsible for Egor's connection to Celty Sturluson. However, it is Shinichi's teasing about Shizuo Heiwajima that ultimately causes Izaya to threaten him before signing off.

Adabashi Arc
Shinichi gives Mikado information on who is misusing the Dollars' name, as per Mikado's desire to 'clean up' the organization. It is revealed in flashbacks, that when Ruri Hijiribe is later searching for her father's whereabouts in the past, he informs her gratis that Yodogiri's men killed her father three years ago. This information serves as the catalyst for Ruri becoming the serial killer 'Hollywood' in a quest for vengeance.

Final Curtain Arc
Shingen Kishitani, via his Twittia account 'Kisshi', divulges several Nebula trade secrets on the grounds that no one is paying attention. When he considers the possibility of a being with a 'superior existence' and 'humanity as its brain', Shinichi - via his Twittia handle 'Tsukku' - remarks in a reply that such a being may already exist.

Initially spooked, Shingen is mollified once 'Tsukku' identifies himself as Tsukumoya - though he wonders how Shinichi contacted an unlisted account he is not following. Shinichi assures him he is no deus ex machina - even as he changes the tweet's font - before warning him that Seitarou Yagiri has made a move to capture Namie Yagiri in pursuit of Celty's head.

Following the main events of the series, Shinichi shuts down the Dollars website.

Trivia

 * Walker admiringly calls Shinichi a 'professional' due to the amount of books he has published.
 * Kasane Kujiragi considers Shinichi to be her second-biggest obstacle, with Awakusu-kai being the first.
 * He is never directly mentioned in the anime, though he has a few unnamed mentions.
 * His only reference in the anime is in an OVA episode when Izaya uses his name a pseudonym and later admits he only chose it to harass Shinichi.
 * 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.
 * 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, which was inserted into the middle of the fourth light novel.