The Rats are a group of children led by Nejiro Kanata, and normally congregate in The Pits. However - as Yakumo Amagiri emphasizes - they can be found everywhere, infesting every little nook and cranny of the Artificial Island.
Notably, all of the children have been abandoned by their parents or guardians - just like Nejiro had once been. Though a few individuals attempted to rebel when they first joined the group, eventually they (like all of them) were 'broken' - they stopped thinking for themselves - more accurately, they ceased being able to think for themselves at all. They entirely depend upon Nejiro to tell them what to do and how to survive.
The Rats all dress in shabby grey clothing, and their eyes are described as "empty." Their emotions appear to be repressed to the point of nonexistence; they know the concept of happiness and desire it, but they do not seem to know what it feels like. They are desensitized to violence - often placing bets on how long it will take for someone to die. Most of them are under fifteen years of age. They talk in monotone statements, usually agreeing with one another over simple facts, observations and questions. As Yakumo puts it, the way they talk is as if they are one entity.
In 2019, Nejiro (courtesy of Ginga Kanashima) equips the children with the Rat, a new small handgun yet to be released to the public. The Rat has poor accuracy, but it is deadly at close range. He plans to use the Rats to eliminate the executives of the Eastern and Western Districts (something that Ginga also wants), which he considers the root of the island's evil and misfortune--and he wishes for the island's destruction.
It is revealed that Nejiro is working for the Western District during the Rats' murders of eight Western executives. When the Rats are given their final mission (to attack the Eastern District's casino), Nejiro secretly warns the Eastern District in advance and 'sells them' the Rats, along with his Western District contract.
(The casino scene plays out thusly: the Rats [sans Nejiro] swarm the casino interior, and aim their handguns at Gitarin, his human shields and the other executives. To the children's surprise, their guns do not fire. They are quickly subdued by the Guard Team and others).
In the aftermath, Gitarin (or someone else) breaks Nejiro's legs in multiple places. Nejiro is found by Yua Kirino, and she brings six Rats to speak with him. In the ensuing conversation, it is clear that the Rats are lost without Nejiro's guidance: their emotions so repressed that they cannot even feel anger at his betrayal. Nejiro keeps his position as 'king' of the Rats, and he decides to undo all the damage he did to the other children and teach them about hope and emotions once more.
Over the ensuing year (continuing on into Summer 2020), the Rats aid Yua in expanding her maps - and in return, her maps are of a great help to them.
That summer, Ginga contacts Nejiro and threatens to kill a couple of the children if the Rats do not do as he says. Under Ginga's bidding, Nejiro and the rats install bombs on the engine in the engine room, and they are armed once more. Essentially, Gitarin takes the Rats' 'home' hostage in order to give orders to Nejiro. When Gitarin catches wind of the situation, he does the opposite: he takes Nejiro hostage to command the other Rats. The Rats are accordingly instructed on what to do in the engine room by Gitarin.
Later in the engine room, the Rats witness Charlotte and Sherlock Liverpool threatened by Lihuang Ei and Lilei Ei (and Jun Sahara attempting to stop the Ei siblings).When Ginga appears, the Rats draw their handguns and bowguns and aim their weapons at him, much to Ginga's surprise (Gitarin points it out to him). Ultimately, they do not have to do anything further.
- One member of the Rats is a Spring-Heeled Joplin.
- In 2021, the Rats get into trouble in the Western District alongside the Iizuka children. The details of this incident may come to light should a second volume of short stories ever be published.