Rengoku feels it in his gut the moment he steps onto the train, he just knows he's getting into more than he signed up for.

When the younger slayers show up Rengoku almost writes that feeling off and assumes they're the cause, but it still doesn't feel right. He's heard whispers about Tanjiro's foray into fire breathing, or fire breathing adjacent. Still, Rengoku's the closest they'll get to an expert. The young man is a sweet fellow with great potential, both as a slayer and a person, so Rengoku happily offers to take him on as a tsuguko. He finds himself looking forward to it once the boy readily agrees.

If Regoku's honest, he's also quite excited to get to know the younger Kamado as well. It hasn't come up with Tanjiro yet, but he and Nezuko share a…. condition, to put it simply.

Rengoku vividly remembers his shock when the siblings were brought before the hashira not long ago, and Nezuko pulled from her box. Rengoku's been controlling himself for many years now, and even he still feels the pull to Sanemi's blood sometimes. He'd been terrified for the girl, but such relief flooded him when Nezuko turned away that the hashiras next to him had given him a concerned glance he'd had to quickly wave off.

Tanjiro's confidence in his sister had been truly inspiring that day and Rengoku can see the care Tanjiro treats his sister with even now as she sits with them on the train, drawing in a small book her brother handed her a bit back when she started getting bored.

Rengokus condition has long been a sticking point with his family. Well, really only with his father. Senjuro really doesn't care, and with his little brother believing in him, how could Rengoku ever let him down. Father, on the other hand, had all but disowned him when he'd turned. In fact, Rengoku suspects the only reason his father hasn't gone through with any of his many threats is that then he'd have to take care of Senjuro himself. Which would be a distraction from his obviously much greater duties to alcohol. It's good that Nezuko has someone looking out for her, heaven knows Rengoku himself could have benefited from having someone like Tanjiro in his life.

He grins a little, knowing his definition of a little means he's beaming, thinking about how the Kamado siblings would get along like a housefire with Senjuro. His brother might not be a demon or a slayer but everyone needs a support system and right now Rengoku is really all Senjuro has. Despite how much Rengoku tries to be there for his brother, his job is dangerous and time-consuming. Introducing them might be good for all of them, the Kamados and their friends still deserve to be children too.

All in all, Rengoku's quite glad to have run into the eccentric group of slayers on this mission.

Rengoku realizes he's fallen silent while lost in his thoughts when Tanjiro calls his name. Quickly coming back to himself, he finds himself being observed by both siblings. He makes sure to spread a blinding smile across his face and tells them he was lost in thought over the bentos he brought. Tanjiro seems to accept the excuse, but Nezuko stares a little longer. Rengoku can see the moment she puts it together. She must have been such a clever human, he gives her an even wider smile and the slightest of nods he can manage. She blinks a few times before smiling back as best she can with the bamboo in her mouth and goes back to drawing without a sound.

They continue a lively conversation for a while before suddenly being reminded of their purpose on the train and must leap into action. It turns out to be quite the event but they successfully liberate the train and take out the demon that's been causing the disappearances. Rengoku finds himself truly grateful for the other slayers' presence, but even as a group they cannot prevent the train from being derailed and themselves from being scattered.

Rengoku finds Tanjiro first, the boy is clearly injured but after a quick assessment, Rengoku figures that after a visit to the butterfly estate he'll be well and good again. Rengoku never gets the chance to look for the others before a demon leaps out of the darkness and lunges for Tanjiro's head.

Of course, dismissing his gut feeling as a response to meeting up with the boys and Nezuko would have been foolish. Rengoku quickly puts himself between his tsuguko and the incoming demon and that's when he feels it.

Rengoku's not stupid, not by a long shot, and he never would have made it to hashira let alone stayed one as long as he has if he wasn't observant either. He's noticed someone watching his missions for months now, he's felt eyes on him as he fought. He's figured it was a demon too as his observer only comes at night, so when he meets this demon's gaze he recognizes the feeling of those eyes on his skin.

Rengoku almost laughs the first time Akaza asks him to become a demon. He will admit it's nice to finally have a name and face to the presence he's felt but he manages to hold in his laughter, if just barely. He can't stop the surprise from showing on his face though. Nezuko'd figured him out in their incredibly short time on the train, and while Nezuko isn't just another demon Rengoku would have figured an uppermoon, let alone uppermoon three, with much longer to observe him would have figured him out too. Either way, Rengoku's not one to look the gift horse in the mouth, so he quickly turns the demon down and dives in to fight.

The ensuing battle is one of the most intense, if not the most interesting battles Rengoku's had in a very long time and he finds himself enjoying it. That, combined with the amusement he feels whenever Akaza prods him to become a demon has him grinning like a madman. Still, Rengoku is grateful to see Inosuke arrive out of the corner of his eye, even if his presence complicates things.

Rengoku knows Inosuke is incredibly capable and will make sure Tanjiro gets out of here perfectly fine, much like Rengoku knows Zenitzu and Nezuko will undoubtedly do the same for each other, despite how Tanjiro worries. Rengoku also knows that to ensure Tanjiro and Inosuke's survival he has to end this fight now. Pulling back he prepares to make his final charge, yelling to draw Akaza's attention back from where it's wandered to the boys.

Akaza might not know who Rengoku is, but Rengoku sure does. He is Rengoku Koyjiro. He is the flame pillar. He is a protector of those around him, and yes he is already a demon but he will never abandon the people who rely on him. With everything in place, Rengoku breathes carefully and charges Akaza one last time. He can feel the fire in his breath filling his lungs and roaring through his body. Rengoku loosens the suffocatingly tight control he keeps his demonic strength the slightest bit. This ends now. He puts everything into his swing not bothering to defend himself against anything. Even as he feels Akaza's fist plunge through his body he brings his sword down again pushing it into the uppermoon's neck.

"You were supposed to dodge" Akaza's voice takes Rengoku by surprise. Mostly for the undeniable anger in it. Why would Akaza be angry? He's winning or would be if Rengoku was human as Akaza thinks.

Rengoku makes eye contact with the demon as Akaza looks up as well, or he makes as best eye contact possible with only one eye. There is a storm of feelings raging in Akaza's eyes, and Rengoku finds himself puzzled. He doesn't get a chance to start untangling the storm in front of him before he's distracted by the incredibly unsettling feeling of Akaza's arm tensing below what remains of his ribcage as the demon clenches his fist. Presumably, that is, Rengoku can't see Akaza's hand by nature of it being, well, through him but that's what he assumes the demon is doing.

"Become a demon and live" Akaza insists again, but this time his voice wavers in a way that snags Rengoku's attention. He sounds choked, almost like he's on the verge of breaking. Maybe, Rengoku thinks, this demon is much more complicated than he'd previously thought. He almost feels sorry for Akaza, something about the desperation in his voice catches Rengoku unawares and he knows his reaction shows on his face as he looks back at Akaza.

"No" Rengoku answers again and this time the emotion in Akaza's eyes is clear as day.

Pure rage burns into Rengoku as Akaza tries to shout "Why" but is cut off sharply to a stuttering almost sob that Rengoku is far too familiar with.

Rengoku's seen plenty of grief in his life and experienced far more of it than he'd like as well. It comes with the life he leads. He's heard that exact question, that exact voice many, many, times before. He heard it when his mother passed. He heard it when Shinobu's sister was killed. He hears it every time they can't save everyone. The sound of grief and loss and desperation so overwhelming you can't breathe. The sound of someone clinging to anything they can, and trying to bargain the life back into someone they love. But why would Akaza feel like that over him? Why would Akaza sound like the husband he'd had to tell his spouse wasn't coming home ever again or the younger sister of a woman who hadn't made it through the night begging the universe to change its mind knowing it never would.

"Why not? " Akaza sobs again, and sure Rengoku's seen demons cry before, but never like this. Never this real, never for someone like him. But sure enough, there are tears streaming down Akaza's face and dripping off his shaking jaw. All of him is shaking, Akaza's body shuddering with what can only be described as sobs "Why would you choose death? You were supposed to dodge!" Akaza shouts again, and suddenly it makes sense.

Akaza never wanted to kill him. He's lonely, probably has been for a very long time and he's lost. Demons, outside some very few special cases, aren't a family. Despite what Muzan calls himself, he's no father. Distantly Rengoku hears Akaza begging again, babbling as tears continue running down the lines on his face.

The sun is starting to rise.

Rengoku knows lonely, he knows fear far too well, so he makes a decision. And maybe it won't work. Maybe it's too much to ask, but maybe, just maybe, Akaza can be healed. Rengoku just needs Akaza to ask one more time, and he does. Akaza chokes back his pleas and tears and whispers "why not. Why can't you stay with me"

Rengoku opens his mouth to speak but ends up coughing up some blood that's pooled in his lungs before he can grit out "You can't become a demon twice" and hopes Akaza understands.

Akazas face contorts in what Rengoku assumes is confusion and maybe anger, then jerks at his arm, still buried in Rengoku's stomach, harshly making Rengoku gasp at the sharp pain, his body flinching to try and escape. Akaza tugs again, harder this time and rips himself away from Rengoku who bites back the scream building in his chest and feels his sword snap.

Realizing Akaza has left his arms behind, Rengoku reaches down and gives them a push himself causing them to slip out of his back fairly easily. Tanjiro and Inosuke are yelling behind him but Rengoku keeps his focus squarely on Akaza making sure the other demon is watching as Rengoku lets the holes in his abdomen heal.

He sees the moment it clicks and Akaza goes completely still before repeating " You can't become a demon twice" as if in a trance. Finally starting to understand the truth behind the words. Rengoku almost laughs at the demon's next words, not expecting the sheer awe and vulnerability in his voice as Akaza murmurs " You. You can stay by my side forever?"

Rengoku smiles as bright and comforting as he can manage. He doesn't know about forever and he is most definitely in over his head if this works, but fuck it, he still has to try. Even if the odds aren't in his favor. " Only if you come with me" Akaza continues to gape at him until out of the corner of his eye Rengoku sees the sun start cresting over the mountains.

Akaza runs.

Rengoku sighs as he watches the other demon retreat into the woods then sees Tanjiro run after Akaza sword in hand. Shouting, the boy raises his sword as if to throw it and Rengoku calls out to him making him pause just before going through with it.

"Tanjiro it's okay" Rengoku calls again, straightening up from the crouch he'd been in and letting his haori fall over his shoulders and hair swing forward. It's definitely not the best situation to block out the sun, but he's had to rely on it before and there's plenty of shade around as soon as he deals with this.

Suddenly Tanjiro is in front of him, talking frantically with tears starting to gather in his eyes.

Rengoku smiles at him, "I will be fine, do not fret!" he assures and Tanjiro frowns opening his mouth to protest as his eyes drop to where there should be a gaping hole through Rengoku's body

Tanjiro's gaze snaps back up to Rengoku's face with a puzzled " What?"

Rengoku sighs, feeling the soreness in his fingers that comes with sprouting claws, and knows his pupils have gone slitted in a distinctly demonic way. He holds himself like this for a few seconds before pulling back, his hands and eyes return to a more human-like state as Tanjiro gasps dramatically " Are…" he pauses " Are you like Nezuko?"

Rengoku smiles at him in his signature way but keeps his head tilted forward to prevent his hair from moving too much " Have been for many years now"

Tanjiro blinks at him a few times mouth agape before his eyes go wide all over again and he shouts " Oh! The sun." and starts frantically looking around for shelter. As Rengoku lets Tanjiro bustle him over to a shaded area to reunite with the other three he steels himself for the inevitable onslaught of questions. finds himself smiling comfortably though, he figures this isn't the last he'll see of Akaza.