(Flashback)
"Get lost!" Kagaku said as he glared down at Zenitsu from a tree branch, "you're useless! Any time spent on you is wasted time!" he snapped, before biting into a peach. "But, Gramps said-!" Zenitsu began, only to flinch when the peach was tossed at his head. "Don't go being all familiar with him and calling him 'Gramps!' Our Master is a great person! A former Hashira! And yet, instead of training any real Tsuguko, he's wasting his time on you! Why do you keep coming back?! Why do you insist on clinging here, like a patch of mould?! Your very existence makes me sick!" Kaigaku snapped furiously.
(End of flashback)
Zenitsu knew he should be scared, terrified. He was standing in front of an Upper Rank member of the 12 Kizuki! And yet, all he could feel was cold fury and hatred. "I can't believe you're still alive. Thought you would have died during Final Selection. After all, a useless weakling like you will never be able to change" the Demon said coldly. Tanjiro was incensed by this, and opened his mouth to defend his friend, Upper Rank Demon or not, but Gyutaro put his arm in front of his younger Brother, blocking his movement. Glancing over at the green-haired young man, he found Gyutaro to be glaring over at the Demon, a determined look in his eyes, as he held Tanjiro in place.
"Kaigaku. You should know, that in the past, I would have agreed with you and just run away. I would have been terrified of you, and just given up, because I knew I was about to die," he glared coldly at his former Senior Student, his entire being screaming focus and deadly energy. "But not anymore. Because you did the one thing that I could never forgive," he gripped his sword's handle tightly, as Tengen moved to stop him, "hey-!" he began to shout, but all of the blond's attention was on the Demon in front of him, fury pouring forth from him in waves, "YOU KILLED GRAMPS!" He screamed, shooting forwards in Thunder-Breathing's First Form. Kaigaku drew his own sword, "Blood Demon Art, Thunder-Breathing, Third Form: Thunder Swarm!" Waves of deadly black lightning shot forth towards the Demon Slayers.
The only reason that most of them got out unscathed was because the attack was mostly aimed at Zenitsu. But the blond was already gone. With steely concentration, Zenitsu forced all fear from his mind and dodged the attack with incredible speed. He drew his sword partly from it's sheathe: "Thunder-Breathing, First Form: Thunder clap and flash!" he cried, shooting forwards with deadly speed- but Kaigaku had already moved. "So, you still only know the one move. And here I thought you might have at least started training with the others. But no. one move is still all you were ever able to use, wasn't it?!" the Upper Six cried, sending another wave of lightning at the 16-year-old.
Once again Zenitsu dodged the blast, feet hitting the wall of the Castle, "Thunder-Breathing, First Form-" but he was forced to dodge again. "Zenitsu-" Tanjiro muttered in worry. More black lightning shot towards them, forcing all of them to dodge and block the stray attacks. "Last until morning?! Yeah right! We'll be lucky not to be incinerated!" Gyutaro cried, jumping out of the way of a bolt of lightning, only for one to hit him in the shoulder. "GHAAAAAAAAAA!" He screamed as agony erupted through him. Not only was he electrocuted, but he was also burned. "Big Brother!" Tanjiro cried in horror. Zenitsu heard the scream, and looked over just as another bolt of lightning headed towards the group.
His focus broke, "no!" he screamed, moving to intercept it. But he knew that he wouldn't be fast enough. Suddenly a bluish-white, translucent shield appeared around the group of humans. The lighting slammed into the shield, and seemed to fade into nothing as it did. Ume pushed open her box and climbed out, growing to her normal size. Her palms were pulsing with bluish-white light, the same colour as the shield around them. She glared up at the Upper Six Demon, snarling, with veins bulging all over her body. "Thanks Ume!" Tanjiro cried gratefully, before turning back to Zenitsu, "don't worry about us! Ume can block any stray attack that you miss! Now get him!" he called.
Zenitsu instantly fell into a crouch again, "Thunder-Breathing, First Form-" another black thunder bolt shot towards him, but this time a wave of hot pink flames engulfed the lightning, making Kaigaku growl and glare down at another Demon girl, this one much younger than the other, with black hair that had orange highlights on the bottom half of it and with a bamboo muzzle in her mouth. She held her hands out, palms glowing orange, with veins popping around her body. A man leapt at him this time, and Kaigaku sent a wave of lightning at him, but a throwing dagger embedded itself into the floor at the Demon's feet and absorbed the lighting like a lightning rod.
Zenitsu smiled in gratitude, "Thanks, all of you. And now Thunder Breathing, First Form: Godspeed!" he cried, shooting towards his former Sibling Student- an explosion of lighting erupted around Kaigaku, blowing the Blond teen away, and hitting everyone else. "You pieces of shit! You think that you defeat me? I am superior to all of you! I'll kill all of you right now!" he cried, furious, as black lighting began to build around his body. Tengan and Zenitsu were both immediately on their feet again, and shot towards the former Demon Slayer, however, before either Slayers or Demon could actually do anything, a Biwa sounded, and Kaigaku was shut behind two shoji doors, which faded away as the Demon Slayers flew through them.
Zenitsu landed on the ground, alongside Tengen, as they both just stared at where the Upper Rank Demon had disappeared. "What the fuck?" Gyutaro asked in shock, as Ume and Nezuko helped him and Tanjiro to their feet, while Inosuke stood up. "Is that it? I DIDN'T EVEN GET TO DO ANYTHING!" He cried furiously. "What just happened?" Tanjiro asked, confused. "The Bastard got away, that's what happened" Zenitsu growled, trying to calm down. "Yes, we saw that bit. We're trying to figure out how he got away, and where he went" Gyutaro replied dryly. Zenitsu fell to his knees and punched the tatami mats so hard that his hand went through them, "damn it! Damn it! Damn it! Damn it! Gramps. I couldn't do it- I'm sorry" he whimpered, tears welling in his eyes.
Gyutaro stared at the weeping 16-year-old as he screamed in fury and bitter disappointment. Tanjiro walked over to the sobbing teen. He placed his hand on the older boy's shoulder, "maybe you couldn't take Kaigaku out this time. But in the future- you just need to get a bit stronger, that's all. And I'm sure your Master wouldn't be disappointed in you either.
If anything, he'd be proud. I mean, look how far you've already come! You're amazing Zenitsu, and you'll be even more amazing the next time you two meet. Just bring me with you, so that I can get a blood sample from Kaigaku, okay?" Tanjiro asked with a smile. The blond chuckled slightly, "sure Tanjiro. Anything to help Nezuko" he replied as he wiped his eyes, glaring at the spot the former Thunder-Breather had been. "Next time you're dead, you Bastard" he growled furiously.
Ư*Ğ.Ķ
They were back in the Wisteria House. Tengen was staring down at Zenitsu as Gyutaro got his wound treated, "so. It sounded like you knew that Demon" he said, raising an eyebrow. Zenitsu sighed, "yeah. That was Kaigaku. He used to train under my Master with me. I never knew what happened to him after his Final Selection, because he just disappeared. Hell, I thought that Bastard was dead! Then, a few days before this Mission, I get a notice, saying that my Master killed himself because Kaigaku betrayed the Demon Slayer Corps and turned into a Demon. I didn't know that the Demon we would be fighting on this Mission was him.
But for the first time in my life, I'm not sacred of a Demon. I'm not even scared because he's now Upper Rank 6. All I want to do is kill that Bastard. My Master, that I had always called Gramps, was the only person in the world who believed in me. The only person who saw something worthwhile, in me. And now he's dead, all because of Kaigaku. I have to get stronger. I have to kill him. To make that Bastard pay for what he did to Gramps. No matter what, I will do it" Zenitsu explained, clenching his fists. The Sound Hashira sighed deeply, "alright, pack up, we're leaving as soon as Gyutaro's injury is healed enough" Tengen decided.
Tanjiro looked at him in confusion, as Gyutaro laid on his Futon, "aren't you going to send a message to the Corps about this?" he asked. The man looked at him, "of course I am, but this will save the time that we have to stay here. Now, the rest of you, except for the injured, get moving!" he called, leaving the room.
Zenitsu glanced over at the oldest Kamado repeatedly, seemingly nervous. "What is it Zenitsu?" the green-haired man asked. Zenitsu looked away, "I'm sorry" he apologised. "What for? You didn't hurt me?" Gyuatro asked. "You got hurt because of Kaigaku. I should have been able to stop him. I swore I would, but when I finally get a chance to keep my promise to Gramps, I completely fail, and one of my friends gets hurt. I'm sorry you had to get mixed up in this" the Agatsuma said. "Zenitsu, you didn't "get us mixed up" in anything. We're Demon Slayers, this is our job. If we get hurt, it just means that we need to train harder, so that we don't get hurt again next time," Tanjiro said kindly.
He smiled, "besides, we're friends, right? That means we help each other no matter what" the burgundy-haired 15-year-old said kindly. Zenitsu finally burst into tears and hugged Tanjiro close. Ume exited her box to see what all the commotion was. "Hey Ume. Everything is fine here, don't worry" he reassured his younger Sister. The 21-year-old Demon moved to check her Older Brother's wounds. Gyutaro smiled at her care, "I'll be fine Ume, I just need to rest. But I do have something to do when we get home," he remembered hearing from his Master about how Kagaya sometime had visions of the future. Maybe the younger man's experience with that ability could help him with his own apparent future prediction ability, or whatever it was.
To Be Continued