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," 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"), 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, Fire0breating Bugs are not without their own moral code. 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.

Chronology
To be added.

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.