Arthur Rimbaud (アーサー・ランボー Āsā Ranbō?) was a European spy sent to investigate the Japanese military's mysterious project related to Arahabaki. After the Suribachi City explosion, he loses many of his memories and infiltrated in the Port Mafia under the alias of Randō, rising to the rank of a sub-executive. He was one of the few Transcendents. 
Rimbaud was a tall, pale man of slim build with vacant, tired gold eyes and long black hair that reaches past his mid-back. A long section of his hair fell in front of the right side of his face, alongside a thinner strand over the left side.
Cold no matter the weather, Rimbaud always wore warm winter clothes. Over his ears is a pair of white earmuffs, and around his neck a tasseled, dark red scarf with alternating thick and black stripes. He wears a grey trenchcoat, black trousers, and red winter boots. Of course, he also wears a pair of black gloves.
Appearing as a gloomy individual, Rimbaud always complained of the cold and tried to avoid it as much as possible. He held profound respect towards Ōgai Mori, and as such rose in rank with ease. As a veteran mafioso, Rimbaud had a deep understanding of the driving forces behind the criminal world, and never showed any delight in his actions as a mafioso. In fact, Rimbaud respected lives a great deal, and as such he went to lengths to ensure his opponents never his opponents suffered more than they must.
Rimbaud was far from emotionless, expressing surprise and confusion in Osamu Dazai's antics, and even laughing after he and Chūya Nakahara rightfully accuse him of being behind the predecessor's apparent "revival".
His respect extends to Chūya and Dazai, as he expressed regret in having to kill them for his own gain, but he did not hold back in attempting to kill them in order to achieve his own goals. Furthermore, he seemed more prone to make drastic decisions when pushed into a corner. Struggling with slight amnesia, Rimbaud's ultimate goal was to learn what happened to his close partner Paul Verlaine. For this reason, he took an intense interest in Arahabaki's existence, vividly remembering its burning heat and how its existence exceeded human understanding.
Rimbaud's ability, Illuminations (イリュミナシオン, Iryuminashion?) creates a hyperspace that Rimbaud has full control over. He's capable of absorbing corpses and using them as his ability. As a Transcendental, Rimbaud's ability greatly surpasses that of average abilities. His hyperspace exists outside natural law, rendering For The Tainted Sorrow useless and overcoming No Longer Human's nullification to an extent.
Rimbaud worked as a European spy tasked with infiltrating rival organizations to obtain and leak their secrets. He, alongside his partner Paul Verlaine, received intel of the Japanese government's containment of a powerful entity and was sent to investigate and steal it. However, at the last minute, Verlaine attempted to kill Rimbaud, leading to a fierce battle Rimbaud believed ended in Verlaine's certain death at his own hand. 
The battle alerted military personnel, and the corpse Illuminations harbored at the time was not powerful enough to defend Rimbaud. In a last-ditch effort to save himself, Rimbaud attempted to attack Arahabaki's vessel and absorb Arahabaki into Illuminations and use it as his new ability. However, this disturbance resulted in momentarily splitting the seal over Arahabaki, which appeared in its full power. The calamitous entity caused a great explosion that formed Suribachi City and damaged Rimbaud's memory. Sometime later, he joined the Port Mafia.
Rimbaud first appears as the Port Mafia's sub-executive Randō, who uses his hyperspace ability to restrain Chūya's hands and feet to a chair. Ogai Mori requests he leave, to which Rimbaud is wary of freeing Chūya so easily. In response, Mori questions if he's cold, and Rando admits he is freezing, unable to deal with the cold whatsoever, and Mori convinces him to free Chūya and leave. During their investigation of Arahabaki, Chūya and Dazai arrive at a dilapidated western-style building, where Rimbaud lives. Outside, they defeat GSS members. Inside, Rimbaud shivers in front of a fire, trying to stay warm against the cold wind blowing in. He and Dazai feed the fire with old, broken pillar material and even an expensive art piece. It's elaborated the GSS members wanted to kill Rimbaud to spark internal conflict between Mori's faction and the former boss', as Rimbaud's loyalty to Mori would raise suspicions inside the mafia. Rimbaud notes the GSS has deep ties with the North American Guild.
Chūya and Dazai request Rimbaud to elaborate on his eyewitness encounter of Arahabaki one week ago in Suribachi City, and Rimbaud shivers in a way, unlike a cold shiver. Rimbaud explains the Port Mafia and Sheep fought once again when Arahabaki suddenly manifested and decimated the area. Despite it vaguely resembled the former boss, Rimbaud describes Arahabaki as a quadrupedal, black beast with flaming fur, a tail, and hellish eyes. Although its figure resembled a human on all fours, something about it was clearly inhuman. Rimbaud viewed the entity as something that “condensed” all times of slaughter and calamity, a materialization of the natural energy holding the world together. Despite his uncertainty of Arahabaki's distinct form, he's convinced that, for all the chaos and age it embodies, it has no distinct, intentional will, and simply existed.
Although he attempted to rationalize the phenomena as an enemy's ability, it exerted such intense, tremulous heat that there was no way a person could create it. Arahabaki's fiery existence caused massive heatwaves that distorted his surroundings, with only Yokohama's seas untouched. Arahabaki erupted into fearsome, wailing howls betraying no emotion, simply being “a sound that it makes” without any intent behind it. Before passing out at the sight, Rimbaud understood that Arahabaki's mere existence brought about destruction.
Soon after the Sheep dissolve their ties with Chūya, Rimbaud and the two boys prepare a “party” in an abandoned building. After a notably long ribbon catches Dazai's attention, Rimbaud comments that the set-up looks more like a site prepared for murder than a party. Dazai explains it's to celebrate Chūya's “freedom” from the Sheep and proceeds to explain his elaborate ideas for celebration. Dazai then asserts that it'd be best for the Sheep and Chūya to reconcile, given the flaws of their current structure. Rimbaud then receives the pleasure of learning about the “grilled cheese theory” Mori taught Dazai. In a nutshell, the theory relates to all opposing parties reaching the same optimum solution at the same time, rendering its effectiveness null and irreversible. To Dazai, this explains the Sheeps' situation perfectly.
Rimbaud agrees and muses the thought of everyone making a rule to not “eat half-grilled meat” (stop fighting and rid their weapons), as their violence is unnecessary. However, Rimbaud is not so naive that he finds this plausible, as there's always someone that benefits from violence, forcing others to retaliate with violence. Rimbaud views this as the natural way of the underground world. Dazai notes Rimbaud knows a great deal more than him, and Rimbaud points out he only rose to such a high-rank thanks to Mori, whom he feels only gratitude towards. Finally, he asks Dazai who the culprit is.
Dazai easily points the finger at Rimbaud himself, much to the man's shock. He continues to mock how any accused person would react, to which Rimbaud cannot interject, instead of asking how Dazai will refute such protests. Their back and forth goes on until Rimbaud urges Dazai to elaborate on why he accuses Rimbaud to be behind the Arahabaki sightings, and Dazai points out a fatal flaw in Rimbaud's account of Arahabaki.
Regarding Rimbaud's account on the Arahabaki sighting a week ago, he mentioned the seas in his report. However, as a valley formed from a massive, spherical explosion in the ground, Suribachi City has no visible view of the ocean. However, the rest of Rimbaud's account is infallible, with detail too poignant to overlook nor doubt, leading Dazai to believe Rimbaud truly did see the sea. How so, in a valley-like location? Because Rimbaud's account dates back not a week ago, but eight years ago, to the creation of Suribachi City's valley. Happening before the explosion, it's entirely possible to see the sea as Rimbaud described it.
Having been found out, Rimbaud admits he's behind the sightings of Arahabaki and the predecessor's “revival”. Chūya instantly erupts and subdues him, but discovered Rimbaud to be the culprit not because of the sea contradiction, but because all other witness accounts saw the predecessor's visage. Meanwhile, Rimbaud described Arahabaki itself, which Chūya considered impossible. Rimbaud asks if it's because he didn't believe in gods, but Chūya asserts gods do exist, and Rimbaud shouldn't have been able to see one. Hearing someone say they know Arahabaki exists unsettles Rimbaud, and Chūya goes on that Rimbaud had to have seen it eight years ago, given his accurate recalling of it.
Rimbaud urges Chūya to tell him where Arahabaki is, mentioning its sheer destructive power, its existence beyond human comprehension and wisdom, and the entity that emitted such heat it burned him, is what Rimbaud is after. After his pleas, Chūya reveals he himself is Arahabaki. The revelation does not shock Rimbaud whatsoever, in fact, he admits he suspected that to be the case since the beginning. Chūya elaborates on how he came to be, gaining a personality and will of his own, and recalls a mysterious hand 'pulling' him from the blue-black darkness Arahabaki once knew alone. He pursued the case to find out where he was found, why he was pulled out of the darkness, and how Rimbaud saw Arahabaki's true form outside its vessel.
Rimbaud covers his face, body shaking. Not from cold, but from laughter. He agrees to answer Chūya's many questions, summoning his hyperspace so large it captures Chūya and, without coming into contact with him, Dazai as well. Given its power to overcome No Longer Human, it's clear Rimbaud's ability exists outside the plane of normal abilities. Rimbaud reveals he's remembered his true name and intent to kill Chūya recently, and as his ability exists outside natural laws, Chūya's ability is rendered useless as well. Identical to the rumors spreading through Yokohama, the predecessor's likeness appears armed with a scythe, revealing Rimbaud's ability, Illuminations, is able to absorb corpses into his hyperspace and turn it into an 'additional' ability.
Rimbaud explains he was a European spy sent to investigate a high energy life force in Japan as part of his mission. The keyword being European, which Dazai understands to mean Rimbaud is a Transcendental ability user, making him extremely powerful beyond normal abilities. Rimbaud removes an imaginary hat and bows, revealing his true name to the boys, and his mission to kill Chūya and have his corpse replace the predecessor's as his ability.
With the predecessor wielding a physical weapon, No Longer Human is ineffective to nullify the predecessor's attacks, injuring him. Rimbaud laments having to kill Dazai, but he cannot afford to leave Dazai alive to inform the mafia of his betrayal. The battle rages on, but even Chūya is overpowered by Illuminations. Rimbaud expresses no pleasure in attempting to kill both of them and admits that he made a mistake eight years ago, ending up surrounded by the enemy. The corpse used as his ability could not defeat his enemies, and he devised a plan to absorb Arahabaki from Chūya's body after attacking him. However, he only pulled Arahabaki's 'safety net', Chūya, out. Attempting to absorb Arahabaki split it from Chūya's body, resulting in the calamitous being's complete form decimating everything and creating Suribachi City. Dazai negotiates five minutes to convince Chūya to give up, but Rimbaud grants them only two. After the two's planning, the fight continues. Despite overpowering the predecessor, however, they're cornered by Rimbaud's hyperspace once more.
Floating in the air, Rimbaud reminds Chūya he was trapped in this same space once before. For the first time, he mentions a partner he worked with attempting to obtain Arahabaki, but his amnesia allows him only to mention something terrible happened that led to them being cornered, forcing Rimbaud to try using Arahabaki's power for himself. He admits he has no idea what on earth Chūya nor Arahabaki are, as the explosion destroyed any remaining knowledge of the facility's experiments. His goal is to kill and absorb Chūya's corpse to recall those memories and understand everything, along with remembering what exactly happened to him and his lost friend.
Rimbaud's partner was a man named Paul Verlaine, and since eight years ago, Rimbaud cannot recall his whereabouts nor what happened to him. Rimbaud needs Chūya's memories to answer these mysteries. All in an effort to save his dear friend. Upon hearing this, Chūya finds himself viewing Rimbaud in a more positive, respectful light, finding no fault in Rimbaud doing anything for his comrade. This new-found knowledge allows Chūya to concentrate his ability and overcome the hyperspace, slowly making his way to Rimbaud before attacking him. However, Rimbaud is unharmed, retaliating with Illuminations. Just then, the boys' trump card, the long ribbon from earlier, is revealed. Chūya hides it under his clothes, with Dazai holding the other end, nullifying Illuminations' effect on Chūya. When Illuminations attacks, Rimbaud is impaled by predecessor's scythe. The pain and shock cause Rimbaud to regain his memories of that fateful day eight years ago.
Verlaine betrayed Rimbaud, trying to kill him from behind and giving Rimbaud little time to react. The two fought a fierce battle, Rimbaud, as the victor, seemingly killing his own friend. The battle alerted the military, leading to Rimbaud's desperate attempt to absorb Arahabaki. Covered in blood, Rimbaud requests one thing of Chūya: live. Chūya's true origins will never be known. But even if he is just the frame for a calamitous entity, he still has his own will and person. To Rimbaud, all human beings, all of life, are frames to protect the mind, body, and material world. To him, those frames are beautiful. Dying, Rimbaud smiles, noting how strange it is he doesn't feel cold. His last breath recalls Verlaine, who likely felt this same warmth upon death.
Even in death, Rimbaud leaves behind a profound legacy and influence over Chūya. A month later, it's revealed Rimbaud's home was burned down, his belongings thrown to sea, and his body left to the elements before receiving its burial in a rural cemetery. All this for his betrayal against the Port Mafia. Chūya visits and sits atop his grave, calling him troublesome even in death, as the mafia discarded all his investigative records of Arahabaki. Rimbaud's grave is already showing wear, left largely neglected for and forgotten. Chūya admits Rimbaud would've never told anyone about his findings had he survived and promises to visit his grave sometime later.
Upon joining the Port Mafia, Chūya receives a black hat the Port Mafia found in Rimbaud's possession. Mori then explains they learned from Rimbaud's preserved documents the military engaged in the experimentation of combining abilities with separate beings, creating artificial abilities. Rimbaud's documents are kept in Mori's possession, as the boss refuses to grant Chūya access to them unless he becomes an executive, ultimately ensuring his stay in the Port Mafia.
- It's hinted that Rimbaud and his former partner Paul Verlaine were also lovers. In fact, the real-life Arthur Rimbaud and Paul Verlaine were engaged in a romantic affair and lived together for nearly four years.
- Arthur Rimbaud and Paul Verlaine are said to be the real-life Nakahara Chūya's inspiration in French Literature.
- The hat Chūya wears is actually Verlaine's.