Inosuke, Tanjiro and Susamaru charged towards the front of the train as fast as they could. That was where Inosuke sensed the strongest demonic presence and the others trusted his senses implicitly. Once all three of them arrived at the train's engine, the train's engineer yelled at them to leave until a mass of demonic hands emerged to fend off the intruders.
"It's here! The demon's neck is bellow us!" Tanjiro said as he sliced away several hands.
"Gotcha!" Susamaru said as she dug her fingers into the metal floor of the train. She tried with all her might to force it open; however, she did not possess enough strength to do so and the small wound she created with her fingers was closing around them and she was forced to retract them.
"Let the leader handle this!" Inosuke said as he unleashed a twin strike against the train and revealed the demon's spine. Susamaru stared at him in awe of how he was so easily able to do what she could not. She then watched as Tanjiro attempted to cut through the demon's "neck"; however, the demon quickly shielded himself with the many hands he could generate and soon the wound Inosuke made was healed as well.
Tanjiro quickly thought of a new strategy. "Susa! Get the driver out of her! Inosuke and I will perform a combo attack!" Inosuke liked the sound of that plan, while Susamaru was left disappointed that she wouldn't be part of the assault. Still, she grabbed the train engineer as she was told and moved him to one of the cars Nezuko and Zenitsu were defending. She was also really annoyed as the engineer stabbed her with some kind of tool because he was actually in league with the demon. She quickly knocked him out and threw him into the car. She would have carefully laid him inside; however, she felt he deserved whatever bruises he got from her dropping him for his little stunt. She then jumped back onto the roof of the train only to be tossed into the air as the entire train lurched. From her new vantage point she could see that Tanjiro had cut the engine from the rest of the train and thus had severed the demon's neck. That was great but now she was worried that all the passengers would be squashed by the demon's death throes. Luckily, when she landed back on the train, Rengoku appeared and announced that everyone was safe with minor injuries. She and him then met up with Tanjiro and Inosuke, who had fallen off the train after successfully decapitating it.
As she met back up with her friends, she couldn't help but feel like she contributed nothing to the mission. She may have saved a single human of the two hundred but she couldn't wake her friends up or damage the Kizuki in any way. It was very different from her last one and she was shocked how strong her friends had become while she hadn't gained any strength of her own. She felt very discouraged until Rengoku slapped her on the back to gain her attention.
"You feel like you didn't do enough right? Well you didn't and you never will!"
Her expression fell even more at his words. He still had his smile on his face and she thought maybe, just maybe he was going to say something encouraging. "I.." she started before he interrupted her.
"That is because no one can ever do 'enough'! We can only do what we can! That is why we strive to get stronger every day so that we can do more! Don't be discouraged! Keep growing stronger!"
Susamaru's sadness was replaced with surprise. This top Demon Slayer was actually encouraging her! As she stared at him in awe, she realized he had the same kind aura that Tanjiro had. He just had a very different way of speaking than her friend. After giving her that advice, Rengoku made his way over to Tanjiro to chat with him as well. She watched as he commended her friend for mastering the whole "Total Concentration Breathing Constant" thing but also told him he had to go further too. As the two spoke, suddenly a massive crash was heard near the front of the train and everyone around became filled with a sense of dread before they even saw what made the loud noise.
Susamaru and Tanjiro could both smell that a new demon had arrived and his scent was incredibly close to Kibutsuji's! They, along with Rengoku turned to see the new arrival. He was a muscular man with pink hair and tattoos that covered his entire body. His eyes held the symbols for "Upper Moon Three" and everyone realized how strong this particular demon was. Even Rengoku himself was a little worried because he had never faced an Upper Moon before; however, he didn't show his concern. Instead he braced himself for combat and bore a more neutral expression on his face as the demon quickly leapt towards him to attack. Susamaru couldn't even see Rengoku or this new demon's movements. All she could do was sit near Tanjiro as the Flame Pillar defended them from the Upper Moon, who introduced himself as Akaza and offered to turn Rengoku into a demon. Predictably, the Demon Slayer refused and the two clashed in dazzling array of explosions and flames. Susamaru would have called their fight beautiful if the lives of every person around weren't on the line. She wanted to join in; to help Rengoku fight and she could tell Tanjiro felt the same; however, neither of them could bring themselves to move. Rengoku himself even told them to stay out of the fight and that he would handle everything. That wasn't really what stopped them though. They both realized that they were of no help in this situation and that they'd only get in the way. Instead, they just watched as their new friend and mentor defended them and everyone else with skills and speed that they didn't even think were possible up until that moment. They didn't even realize that Inosuke had walked up to them, and was feeling the exact same way, with how focused they were. Eventually, a pause in the fight occurred after a particularly large explosion. When the dust settled, Tanjiro, Susamaru and Inosuke gasped as they saw how injured Rengoku had become. His wounds were really bad and he was bleeding from his side and face. Still he was standing tall and his resolve was apparent to everyone, even the demon he was facing.
"I will fulfill my duties! I won't let anybody die here!" Rengoku yelled as he took a new fighting stance.
Akaza was almost as awed as the humans around Rengoku and begged him to become a demon even more fervently. Rengoku's only reply at that point was to unleash his most powerful Flame Breathing technique. Sadly… it appeared to be in vain as Rengoku was struck in the chest before he could fully strike Akaza. Tanjiro, Inosuke and Susamaru all screamed his name with tears in their eyes as they despaired that he had seemingly lost; however, Rengoku himself was not deterred in the slightest. He remembered his mother and that gave him the motivation he needed to slice into Akaza's neck. He could see how terrified the demon was and he blocked the demon's frantic last strike and held the Upper Moon in place. The sun would soon rise so all Rengoku had to do was hold the panicking demon in place for a few moments. In that moment, Tanjiro finally found the strength to move towards the battle. He encouraged his friends to do the same so all three of them could finish off Akaza but as they got closer, the demon used his ferocious strength to rip his own arms off and break the tip off of the sword in his neck. He then began to run from the sun and Susamaru realized she needed to do the same. Luckily, Inosuke still had her box on his back and she quickly climbed in. From inside, she heard Tanjiro throw his sword at the retreating demon as he proceeded to tirade against the demon's cowardice. She couldn't help but also feel ashamed by Tanjiro's words as she also moved as fast as she could to hide from the daylight instead of pursuing her foe.
As Tanjiro finished his speech he began to wail. He had only known Rengoku for one night but that was enough time with a man so great that he couldn't help but admire the Flame Pillar. He was lamenting the fact that their friendship had only just begun and yet… it was coming to an abrupt end. As he cried, Rengoku called him over for one last chat. As the two spoke, he gave Tanjiro his last wishes and offered words of encouragement to him, Inosuke and Susamaru before he passed. Not long after, Zenitsu arrived carrying Nezuko's box and the group mourned together in their own ways. Tanjiro was full of self doubt just as Susamaru was while Zenitsu was terrified of the power of their enemies. Inosuke begged all of them to stop crying and saying such weak things; however, he couldn't help himself from bawling either.
Eventually, the Kakushi arrived and carried the remaining Demon Slayers back to the Butterfly Mansion so they could recover. Shinobu worked on Tanjiro, Inosuke and Zenitsu's injuries for a couple hours. Luckily, none of them were seriously injured and because of that, Tanjiro left to immediately inform Rengoku's relatives of his final wishes. Zenitsu decided to take some time to rest while Inosuke was nowhere to be found. Susamaru decided to stay in her box the entire day. She felt even more useless than she did after not being able to help against Enmu. Now she had failed to help twice and she was incredibly discouraged. That night, she decided to sit on the roof to be alone in her own depression. She reflected on Rengoku's words to her and she appreciated them but she felt that she would never live up to them. He told her to "grow stronger" so she could save more people and help her friends but… how was she supposed to do that? She couldn't train her muscles like humans could and the only way for her to make herself more powerful was to… eat the very humans she now wanted to protect. In other words she was stuck at her current strength and it made her furious at herself as she cursed her demonic nature even more.
"Hey, is everything alright Susamaru?"
The temari demon's head shot up to look at whoever was speaking to her. She saw the kind but sad gaze of Shinobu and at first she wanted to tell her that everything was fine but… she couldn't bring herself to say a word and instead began to cry once more. Shinobu sat next to her and began to hug the sobbing demon until she calmed herself down. After Susamaru's crying started to lessen, Shinobu offered her a glass of blood tea which Susamaru accepted.
"I admit I'm surprised Rengoku's death has affected you this much… but then again he had a knack for befriending everyone so quickly. I'm sure everyone he ever met is deeply saddened by his passing," Shinobu said.
"Yeah… I'm sad that he died… but that isn't really why I'm so upset," Susamaru admitted.
"Oh? Then what vexes your soul?"
"I… I was so weak! I couldn't do anything! I couldn't even scratch the Lower Moon, let alone an Upper one! I could only watch as my friends struggled and Rengoku died! How am I supposed to live with myself after that?! What am I supposed to do?!" Susamaru said as she began to sob once more.
"Ah, I see. I've seen many before you go through this exact same thing. Truth be told, I felt this way after my sister died," Shinobu admitted.
"You did? W-what did you do?" Susamaru asked as she looked at her friend once again.
"I trained until my bones broke! I wanted to get strong enough so that I would never lose anyone I cared about again!"
"Oh.." Susamaru said as she gazed towards her feet. Shinobu saw that her answer had not helped Susamaru and she had a feeling that she knew why. Demons didn't typically train to gain their strength and Susamaru probably didn't realize that there was something she could do to grow stronger without taking lives.
"You know… there is a way for you to train Susamaru," Shinobu said.
"There is?!" Susamaru said as she sat up straight. A flicker of hope had been ignited by Shinobu's words and she was desperate to cling to it.
"Yes! You can learn Total Concentration Breathing just like the rest of us!" Shinobu said as she clapped her hands together.
"That weird breathing thing you guys do? Does that actually make you stronger?" Susamaru asked confused.
"Yes! That's how we can fight at the level of demons! Why don't I set up a demonstration?"
"Okay," Susamaru said as she became curious.
The two then left the roof and Shinobu searched for someone to help her make her point. Eventually she stumbled upon Inosuke training in the yard… despite her explicitly telling him not to. Still, he was a good choice for her demonstration so she decided to be mad at him later and instead beckoned him over.
"Inosuke, could I borrow you for a moment?" Shinobu said in a sing-song voice.
"Sure!" He said as he ran up to her and Susamaru.
"I want you to arm wrestle Susamaru for a few minutes."
"Can do!" Inosuke said excitedly. He was eager to find a distraction from dealing with his grief and arm wrestling one of his friends sounded like a great one. The three of them then went inside and found a suitable table. Once they acquired one, Shinobu walked the two others through what she was going to demonstrate.
"Alright, for the first round, Inosuke, I want you to just use your muscles! No Total Concentration Breathing! Got it?"
"Of course! My muscles are really strong! I don't need Breathing to win!"
"Great!" Shinobu said as she told them to ready themselves. Susamaru and Inosuke locked hands and waited for Shinobu to tell them to begin. Susamaru was still very confused about what was going on but she was going to do her best anyway. "And begin!"
Almost immediately veins popped out of both competitor's biceps and forearms as they struggled against one another; however, it quickly became apparent that Inosuke was fighting a losing battle as Susamaru quickly gained the upper hand and, after ten seconds, she was able to slam his fist into the table.
"Wonderful! Now, for round two! This time I want you to use Total Concentration Breathing Inosuke."
Inosuke simply grunted in response. His pride was a bit hurt at losing the first round; however, he definitely wanted to regain it and so he locked hands with Susamaru once more and implemented his breathing technique. This time, the outcome was very different. Within a single second, Susamaru's hand was slammed into the table as Inosuke used his breathing to strengthen the muscles in his arm.
"W-what?!" Susamaru exclaimed. "H-how?!"
"See? That's the difference between a normal human and one using breathing!" Shinobu said. "Now that you've seen that it actually works, would you like to learn it?"
"Yes!" Susamaru said immediately.
"Perfect! We'll start tomorrow night!"
Susamaru had no idea what she was in for but she was glad to have a solution to her predicament and over the course of several weeks, Shinobu rigorously trained the demon in the ways of breathing. Unfortunately, without a sword it was difficult to teach her a specific breathing style so she could only be taught the basics; however, Shinobu was confident Susamaru would still make great use of such basic concepts such as strengthening one's muscles or legs. As Susamaru understood breathing techniques more she was even able to better tailor them to her demonic form as she was able to use them to speed up her regeneration. Shinobu was pleased with her progress and after a month of training, Susamaru began working on Total Concentration Breathing Constant so she could be on the same level as her friends. As she was training, so too where they except they were simply fine tuning what they already knew in between missions which she would accompany them on occasionally so she could test her new abilities.
She was very happy to discover that after three and half months of training, she could keep up with her friends and was able to use her skills and powers in conjunction with their swordsmanship to kill lesser demons faster than ever. Her hard work was paying off and she hoped that someday she could live up to Rengoku's expectations of her. She saw that her friends all felt the same way and so they all trained until their bodies were pushed to the limit until one day, four months after Rengoku passed, they arrived from a mission only to find someone trying to kidnap several of the girls from the mansion!
"What the fuck?!" Susamaru yelled as she and her friends charged towards the swordsman who was taking Aoi and another female assistant. Little did any of them realize that this chance encounter would lead to their most dangerous mission yet...