Paul Verlaine/Synopsis

Fifteen Arc
Following the Arahabaki Incident, Verlaine infiltrates the upper floor of the Port Mafia headquarters to steal the files on Chūya Nakahara and retrieve Arthur Rimbaud's hat. In the process, he murders the security who were guarding Ōgai Mori's office and destroys the window to escape. Later, he voices out his intent to take Chūya with him, referring to him as "younger brother".

King of Assassins Incident
One year later, Verlaine assumably encounters Europole detective Adam Frankenstein, whose aircraft eventually crashes in a forest. Later, during Chūya's first-year Mafia anniversary, Verlaine ambushes and assassinates the five Flags sans Chūya. Disguising as a Mafia mailman, Verlaine retrieves Chūya in an attempt to invite him in accomplishing various endeavors together as "brothers". The two, also later with Adam, break into a skirmish after Verlaine reveals having killed the Flags.

During the fight, Verlaine tells Chūya that the boy, like Verlaine himself, is not a human but a mere string of codes. To prove his revelation, Verlaine opens Chūya's Gate, unleashing Arahabaki for 0.3 seconds and ravaging the surrounding area. Verlaine also declares his plan to kill everyone who matters to Chūya to grant him freedom, then departs after Osamu Dazai arrives and closes Chūya's Gate with his ability.

Later, Verlaine visits Dazai from whom he retrieves internal Mafia files they had negotiated over two days prior. He soon storms the police station, where he incapacitates Adam and questions Buichirō Shirase about Chūya's past. Thinking that Shirase is one of Verlaine's targets, Chūya rescues him from Verlaine, and the two battle each other once again. Ultimately, Verlaine spots the detective Murase and kills him instantly, being his actual first target, and retreats.

For his second target, a researcher named N, Verlaine breaks into a secluded military facility, where he finds Chūya being tortured by the researcher. Verlaine releases Chūya from his restraints and urges him to kill N, but Chūya "double-crosses" him and, impaling him with a metal stake, voices out his animosity towards him despite their shared antagonism towards N. They fight once more, and once Verlaine attempts to murder N, the latter mentions knowledge of The Secret of the Gentle Forest, forcing Verlaine to keep him alive for questioning.

Verlaine demands him to tell everything he knows about The Secret of the Gentle Forest, which apparently contains knowledge surrounding Verlaine's origins, but N dodges his questions and slyly brings down upon him a skeleton of Chūya's now deceased clone, controlled by N's technology. Disgusted by what N had done with the boy, he apologetically blasts off the skeleton and chases N down. He then blasts out of the facility with N and interrogates him atop a tower crane. Once he finds no use of N, he leaves him hanging on the crane and departs to kill his final target, Mori.

He spots the train carrying Mori and derails it, only to find that "Mori" is a decoy, doubled by Ryūrō Hirotsu. With the plan masterminded by Dazai, Verlaine clashes with countless mafiosi, ability users, and other forces, forcing him to activate Brutalization and unleash his singularity, Demonic Beast Guivre. He decimates most of the fleet, until Chūya and Adam poison him, being one of his few weaknesses.

However, Verlaine's Gate opens again when N remotely activates Brutalization; before losing unconsciousness, Verlaine urges Chūya to run and live. As Guivre is unleashed in the battlefield and Chūya and Adam are swallowed inside the beast, Verlaine shows Chūya his memory of nine years ago when Verlaine and his partner Rimbaud stormed the military facility and abducted Chūya. In this memory, Verlaine also shows him how to defeat Guivre.

Chūya taps into Arahabaki's power and fights Guivre; as Verlaine asks Chūya to stop Guivre, Arahabaki Chūya unleashes his energy on the malevolent beast and finally defeats it. Verlaine then loses most of his ability and dies. However, he soon finds himself in Rimbaud's hyperspace; Rimbaud—using his ability to turn humans into abilities—turns himself into an ability, generates a self-contradicting singularity as he indefinitely repeats the process, and gives the singularity to Verlaine as his new singularity in place of Guivre, while Verlaine remorsefully apologizes to his former partner.

In the aftermath of the incident, Verlaine allows himself to be locked up in a Mafia basement, unable to flee from European authorities after him and finding no interest in the outside world. With his skills as an assassin, he mentors elite mafiosi, including Kyōka Izumi, Gin Akutagawa, and countless others who would become first-class assassins during their brief training. Eventually, he is promoted to the rank of Mafia executive, still keeping himself locked up in the basement while awaiting "a storm".