You and the girls went to the Butterfly Mansion with the others. 

Butterfly Mansion (Home of Shinobu Kocho!) 

???: Okay, listen up! Here's how training works here! 

Aoi Kanzaki (A helper at the Butterfly Mansion! She is very strict and standoffish, but also kind and caring!) 

Aoi: First, these three will loosen any of your stiff muscles. 

Sumi, Kiyuo, and Naho (Servants of the Butterfly Estate!) 

Then you will face Kanao here in reflexes. These cups here contain medicinal tea that can be applied topically! You will compete to see who has to take the medicine. If your opponent holds down your cup before you can raise it, then you have to drink it. Then there's full-body training. We play tag. Your opponents are me and her. 

Kanao Tsuyuri (Shinobu's adopted sister! She represses her emotions, and fails to do things on her own due to suffering a lot of abuse as a child!) 

(Y/N): Huh. This is quite interesting. 

Zenitsu: INTERESTING?! THIS IS HEAVEN! 

Team RWBY: EXCUSE ME?! 

Tanjiro: Zenitsu, don't say that out- 

Zenitsu: ARE YOU KIDDING ME?! WE GET TO TOUCH THE GIRLS! THEYRE GOING TO MASSAGE OUR BODIES! WHEN WE PLAY THE CUPS, OUR HANDS WILL TOUGH! THEY BOTH HAVE NICE CHESTS, GREAT BUTTS, AND- 

You knocked him out. 

(Y/N): This is going to be embarrassing. 

Several sessions later... 

You always beat both Kanao and Aoi in both reflexes and tag. 

And well, they were both blushing. 

(Y/N): Too fast for you, Cutie? 

Aoi: (blushing) Oh ... my. 

The girls were also on top! 

Though as for Zenitsu! 

Zenitsu grabbed Aoi! 

Zenitsu: WAHOOOO!! 

But then- 

(Y/N): ZENITSU! 

Zenitsu: AAAAAAAAAAAAAAAAAAAAAAAGH! 

You gave him a bloody nose! 

Ruby: Poor Zenitsu. 

Weiss: He deserved it to be honest. 

Blake: Agreed. 

Inosuke on the other hand, well- 

Inosuke: HAHAHAHAHAHA! I WIN! 

Yeah, he was determined not to lose. 

Later... 

You were training Tanjiro. 

Tanjiro: (Y/N), I can't thank you enough for standing up for me. 

(Y/N): Think nothing of it Tanjiro. To be honest, we need more people as kind-hearted as you. 

You then got angry thinking of Sanemi. 

(Y/N): Kagaya must see some good in him. There must be a reason why Sanemi hates demons so much. Even so, how he treated Tanjiro and Nezuko ... I can't overlook. 

Tanjiro: Uh ... are you okay? You seem quite- 

(Y/N): Yeah. I'm fine. 

Sumi: Mr. (Y/N)? 

(Y/N): Huh? 

Sumi, Kiyo, and Naho appeared. 

Kiyo: We wanted to bring you and Tanjiro these. 

They brought a bunch of gourds over. 

Tanjiro: What are those? 

Naho: When Shinobu trained Kanao, she made her blow into those gourds. 

(Y/N): Oh, is this for Breathing Styles? 

Naho: Yep! You're supposed to burst them! 

(Y/N) and Tanjiro: B-Burst? 

Much later... 

You were sitting on the roof at nighttime. 

???: You know, I really like you, (Y/N). 

(Y/N): HUH?! 

Shinobu appeared next to you. 

(Y/N): Sh-Shinobu! How did you know I was here? 

Shinobu: Ruby mentioned that you like to meditate to yourself. 

(Y/N): I'm trying that total concentration thing that Tanjiro is working on. But, I have to wonder, why'd you let us stay with you? 

Shinobu: Well, I really like you. You're a kind-hearted person. I can tell. 

She blushed. 

Shinobu: He's so cute. 

(Y/N): God she's cute! 

Shinobu: I want to get along with Nezuko. 

(Y/N): You know, you're always smiling. And even though you're a sweetheart. I can see a lot of anger deep down there. 

Shinobu: Yes. A demon murdered my older sister, Kanae. So, whenever I see a demon or people who have lost loved ones because of demons, my anger grows stronger. I just can't stand them. It's the same for the other Hashira. Now that we've seen Nezuko, who refuses to eat anyone, I know about her true nature. My sister, was a kind person like you. Even as she died, she pitied them. I couldn't think like her. Pity someone who kills people? But if that was her wish, then I must try to carry it out. If there's a way to avoid killing pitiful demons, I have to try without losing my disposition. But it tires me out. Demons always lie to save themselves, just on instinct without reason they kill people. 

(Y/N): Wow. Shinobu, I- 

Shinobu: (Y/N). Keep fighting. I want to see you, Tanjiro, and everyone else get strong. 

She left, but not before giving you a big hug. 

(Y/N): (blushing) Wow. What a woman. 

Meanwhile... 

Infinity Castle (Muzan's Base of Operations!) 

All of the other Lower Moons gathered together. 

And before them stood- 

Muzan Kibutsuji (The Demon King! The bastard who turned Nezuko into a demon! Leader of the Twelve Kizuki!) 

Lower Moons: L-Lord Mu- 

Muzan: WHO SAID YOU COULD SPEAK?! 

They all bowed down in fear. 

Muzan: Rui is dead. Tell me, why are all of you so weak? Joining the 12 Kizuki is not the end, it's merely the beginning. You must eat more people, get stronger, and become more useful to me! For more than a century, the members of the Upper Moons of the other 12 have not changed. The ones who put the Demon-Salying Hashira in their graves are always in the Upper Moons! But what about you? How many have those members changed? Every time you encounter one of the Hashira, you want to flee! 

Lower Moon 2! No! I don't I risk my life for- 

Muzan: DO NOT CONTRADICT ME! 

He kills Lower Moons 2, 3, 4 and 6! 

Muzan: Enmu, do you have any last words? 

???: Yes. 

Enmu (Lower Moon 1) 

Enmu: Thank you mu lord! I revel in the misery and suffering of others! I relive it in my dreams each night! Thank you for saving me for last! 

He then injected him. 

Muzan: I like you. I think giving you my cells will make you stronger. Now, kill a Hashira. Alright? And also, go after the Demon Slayer with Earrings that look like Hanafuda Cards. If you do, I'll give you more blood. 

Back with you guys... 

(Y/N): So, you're saying that it was like fire? 

Tanjiro: Yes. It was called Hinokami Kagura. My father used something like that. 

(Y/N): Hmm ... I think one of the Hashira has the ability to use flames. Shinobu told me about this guy ... Kyojuro Rengoku. I suggest we talk to him about it. 

Tanjiro: Okay!