Fire-breathing Bug

Fire-breathing Bug (火吹き蟲 Hifuki Mushi) is a "troublemaker"-classed collective of people responsible for multiple acts of arson and murder committed in Shinjuku over the course of a century. Bug believes that anyone or anything connected to the Other Side of the Sky is a "bug" in Earth's system that necessitates extermination, including themselves and especially the Bastard Children of Sabaramond: an organization against which Bug has a century-old vendetta concerning its experiments on human children.

At the scene of their crimes, Bugs leave the following message as their calling card: "This world is a buggy program" (この世は真っ当にあらず Konoyo wa mattō ni arazu), followed by a second half that varies depending on the context. Only the first half—"This world is a buggy program"—is known to and publicized by the media, as Comps-3 has kept the second half a secret during its ongoing investigation into the troublemaker.

Collective
The individuals—hereafter referred to as "Bugs"—who constitute Fire-breathing Bug have no commonalities in ethnicity, gender, age, class, profession, or other demographic criteria. The primary signal that identifies or unites Bugs is their slogan 「火の用心」 (Hinoyōjin, meaning "watch out for fires"), written in the past as 「火乃要慎」, as emblazoned on various forms of rain gear: although dark umbrellas are their signature accessory, a raincoat with the slogan on its back has been an identifier for at least one bug. Caution must be taken before associating all rain appurtenance with all Bugs, as not all may be wearing 'branded' rain paraphernalia, and it is meanwhile only natural for the masses to don rain gear when it rains; moreover, there is no guarantee that Bugs will be able or choose to have rain gear on their persons when they are acting in the collective's name.
 * Identifying Factors

When looked upon from a bird's eye, an assembly of umbrella-carrying Bugs resembles a swarm of black insects.

Traits and Worldview
Due to its traumatic origins, Fire-breathing Bugs view existences like themself to be mistaken on the world. Hence it vehemently despises the Bastard Children of Sabaramond as the group was responsible for bringing them into existence and trying to extinguish them in a fire. They view all those from the "other side of the sky" as enemies, seeing them as having no place in the world and seeking to burn them out in flames. They view themselves as a guardian of the world in protecting it from such outsiders.

Despite their single-minded violence, Fire-breating Bugs is not without their own moral code. While single-mindedly pushing vengeance against those wronged them, Fire-breathing Bug has no tolerance for those who target the innocent, as shown with their interaction with Know, classified him as a "sham, phony bug" for using their name and corrupting their work. While Know was a normal human, the Troublemaker saw him as a rabid animal that needed to be put down for the sake of the peace.

Additionally the Troublemaker, having no qualms about using others as puppets to commit murder, refuses to kill those who seek justice, such as the members of Comp-3. Even when the officers stand in their way, Fire-breathing Bug uses physical force to push them aside rather than immediately incinerate them.

In their interactions with other Troublemakers like Phantom Solitaire, they view the magician as somewhat delusional in his mad desire to seek out the occult and the truth of the Bastard Children of Sabaramond. They compare the magician to a moth drawn to a flame in jest. However, they value Phantom Solitaire as an ally as they fundamentally seek out the truth and cooperate with each other, though Fire-breathing Bug would rather be on the offensive. At the same time, Phantom Solitaire would instead observe and contemplate.

While it may seem that nothing can restrain Fire-breathing Bug's need for vengeance, as they believe no one has the right to question their reasons due to the pain they experienced, they will cease their bloodlust when they encounter those who suffered similar fates as them such as Suzuka Shinoyama who appeared before them to appeal to them to reconsider seeing Corpse God as an enemy.

Modus Operandi
Fire-breathing Bugs wield various emitters that remotely ignite a target with what are speculated to be electromagnetic waves or lasers. Tsubaki Iwanome's working theory of execution, as inspired by Mikoto Saimyouji, proposes that the Bugs use their emitters much like surgeons use gamma rays to conduct Gamma Knife radiosurgery: all Bugs aim their emitters to intersect with the target at their junction; anything that is in the epicenter of the cross-'rays' is burned.

This method can deliver precise results over considerable distances if its perpetrators have perfect accuracy and synchronicity, which the Bugs do; they routinely burn legible letters into significantly remote targets.

Recent developments suggest that Bugs are recipients of song-based brainwashing. Fire-breathing Bug's penchant for singing children's folk songs can be re-contextualized as a deliberate brainwashing technique used to control or influence Bugs within hearing range.

Background
In the past it was mentioned that Lemmings clashed with the other Troublemaker, though admitting only was able to see their shadows.

Lemmings Incident
Tsubaki Iwanome of Comp-3 mentioned Fire-breathing Bug during a briefing with his department, as those troublemakers who were still at large. It was believed by Iwanone that the fire at the Shakuzawa Building was the work of Fire-breathing Bug.

Shinoyama Estate Arson
In the aftermath of Gorō Shinoyama's and Know's failure in killing the Shinoyama Group, Know whilst being transported via a police car was intercepted by Fire-breathing Bug, using their song the thralled the police officers within the car calling Know a sham, before lighting him on fire for his transgression of using their name to commit heinous crimes.

Notable Bugs

 * Mitsu Haigawara and Takajirou Toura (two key founders)
 * The slim, suit-wearing individual who was the first Bug to speak with Sorimura in person, who was seen at Know's murder, and who spoke with Corpse God via cell phone from the stairwell. It is possible that each of these Bugs was unique, but no evidence has been provided either way.
 * The raincoat-wearing teenage girl who had her fortune read at the Fortuneteller Shop and later monitors the Great Shinjuku War.

Trivia

 * The Fire-breathing Bug's moniker is first mentioned at the end of Chapter 5, twelve chapters before their formal debut.