Several Years Later
Zenitsu and Nezuko stood in front of their daughter.
"It's time for your training," Zenitsu said. "And as your father, I'm gonna teach you Thunder Breathing."
Hayami yawned in boredom. "I don't wanna learn your style. I wanna learn something cooler. What other styles are there?"
"I can teach you Blood Demon Arts!" Nezuko chirped.
Hayami shook her head. "There's gotta be something new. Something that's different from all the rest. Hey, Tanjiro, what do you practice?"
"Hey, hey, hey! Hold up, kid! I am as committed as your father is to this relationship, so I would appreciate it if you didn't use my name so casually!" Tanjiro said.
"Fine, fine," said the ten-year-old Hayami. "What do you practice, Dad?"
"Water Breathing," Tanjiro said with confidence. "I also know Sun Breathing, which dates back to the Dance of the Fire God mine and your mother's dad used to do when we were kids."
"Cool!" Hayami said. She assumed a thinking position. "But not cool enough."
"What are you looking at learning? This is all that we're capable of teaching you," Tanjiro said.
"I wanna learn Ice Breathing!" Hayami said.
All three of them blinked their eyes. They've never heard of that before.
Hayami held up a book. "It's in this book of Demon Slayer techniques. It was written around this era, I must say."
"Then it's new," Tanjiro said.
"Damn right, it's new!" Hayami said.
"Hey, don't you swear at us, little girl!" Tanjiro said. "That is uncalled for!"
"I can never yell at a girl. Why the hell did I even become a parent if I can't even yell at my own daughter?" Zenitsu sulked, sobbing. "I'm a terrible father. I never should've had sex with you, Nezuko!"
Nezuko patted Zenitsu's head. Zenitsu looked up at her, his tears subsiding. Now he was only sniffing with snot dripping down his nose.
"At least you understand," Zenitsu said. "You're a kind and beautiful girl. Nothing could ever change that about you, Nezuko-chan."
"Yes. She is a kind girl. Always has been," Tanjiro said. "That's why she's the perfect younger sister."
Hayami looked at the book and grasped her sword. She gave it a couple warm-up swings before getting started. "It looks here that there are twelve forms in total. That's gonna take a while. But if I'm gonna be a Demon Slayer, I'm gonna master 'em no biggie!"
"Does she know that she doesn't need to master all twelve forms?" Zenitsu whispered to Tanjiro.
"She seems like a smart girl. She'll know when to stop," Tanjiro said with a smile.
It was so cute that Zenitsu blushed and toppled over. Tanjiro chuckled, his face also flushing red as he pulled Zenitsu up.
Tanjiro then broke the silence, "Hopefully she knows when to stop…."
Hayami studied the book and mimicked the movements over and over and over again until she felt like she mastered each form.
Closing her eyes, she concentrated on breathing. Then she performed the fighting movements as she had mastered. "Fourth Form: Showers of Hail!"
Tanjiro and Zenitsu watched in amazement.
"She's doing it! She's actually doing it! And she even mastered Total Concentration Breathing too!" Tanjiro exclaimed.
"I can't believe it. I didn't even get it this well on my first try," Zenitsu said, sniffing. "It's a miracle."
"You can say that again," Tanjiro said. "She's amazing! She could become the next Hashira!" He smiled. "I see it now. Ice Hashira Hayami Agatsuma."
"Seventh Form: Absolute Zero Slash!" Hayami called out.
The air around them suddenly turned cold - exactly cold enough to be Absolute Zero.
"It's so cold!" Zenitsu and Tanjiro wept.
"Endure it," said Hayami as she channeled the cold air. She raced towards a nearby tree and violently slashed her sword, eventually cutting it in half.
"Are those all the forms you wanted to learn?" Tanjiro said, still rubbing his arms from the cold.
"Nope. I wanted to learn Forms 2-12," Hayami said.
"That's a lot of forms to memorize all at once. Are you sure you can remember all of those?" Tanjiro asked.
"Wanna bet? How many Water Breathing forms do you know?" Hayami asked.
"Uh, let's see," Tanjiro said. "There's a total of 11 forms, but the 11th form to my knowledge, was created by the Water Hashira, Giyu Tomioka."
"What are those forms?" Hayami asked, the girl growing impatient by the minute.
"First Form: Water Surface Slash, Second Form: Water Wheel, Third Form: Flowing Dance, Fourth Form: Striking Tide, Fifth Form: Blessed Rain After the Drought, Sixth Form: Whirlpool. My version of Whirlpool combines both Whirlpool and the third form, which is Flowing Dance," Tanjiro replied. "There's also the Seventh Form: Piercing Raindrop/Drop Ripple Thrust Curve, Eighth Form: Waterfall Basin, Ninth Form: Splashing Water Flow, and last but not least, the Tenth Form: Constant Flux."
"Wow! And you've mastered all of those?" Hayami said. "I'm impressed."
"It took a while, but I finally got there," Tanjiro said. "And I'm proud of all my hard work."
"Hard work, eh? Looks like hard work ain't everything, Dad." Hayami pointed to her head. "Smarts help you retain information quicker, didn't you know?"
"Of course I knew that! Why else would you already know all these forms without extensive training?" Tanjiro bellowed .
"Quiet down," Hayami said. She clapped her hands together. "I omitted the first form because why the hell would I wanna learn something like Freezing Rain? I first thought: what's the point of this attack? Then you got me thinking. Maybe I should master all twelve forms."
Tanjiro smiled. "If I can master all ten, you surely could master all twelve. You've just got one to go! Good luck!"
Hayami rolled her eyes and began studying the page on First Form: Freezing Rain. After studying everything, Hayami rolled right into practice, grasping her sword with her dominant hand. Hayami fixed her footing and aligned it to the way of the Demon Slayer in the picture. Then, she raised the hand with her sword up to the sky just enough that she could reach the clouds.
"I'm gonna need a demon's heart for this one," she said. "But since it's just practice, I'll practice on this tree over here."
"Soon there's gonna be no more trees," Zenitsu cried. "Then what will we have to practice on?"
"It's OK, Zenitsu," said Tanjiro. "New trees grow every day. Or I thought they do in this area. We'll see, I guess."
In response, Nezuko smiled her usual adorable smile and patted Zenitsu on his head. But this time she ruffled his blonde, French fry-like hair that was caused by getting struck by lightning when he first started training with Thunder Breathing.
Hayami smiled, making her way to the next tree.
"That agility too! For her first time, that agility is fucking amazing!" Tanjiro noted.
"I wish I had that agility instead of being huddled up in a tree and not wanting to come down," Zenitsu said.
"That happened… really?" Tanjiro said.
"Don't laugh! Please don't laugh!" Zenitsu said. "It's not funny! I was terrified, OK?"
"Alright, alright," Tanjiro said. "I won't laugh."
And then, without thinking, Tanjiro leaned in and gave Zenitsu a smooch on the lips. Zenitsu reacted in a full-face blush. So did Tanjiro, who backed away.
"We're gonna have to get used to this," Tanjiro said.
"I'm not even used to kissing Nezuko yet either although it's been about two years since I married the two of you," Zenitsu said. "What can I say? I'm a simp, dammit."
Tanjiro chuckled. "That you are, Zenitsu. That you are."