[Accident, Pantypooping, Soiling, Humiliation]

"Clean. Good Girl."

Those words had been ingrained within Yuzu's mind. Ever since the day that she met Mei. Ever since the day that they became sisters. All because of a simple accident, their relationship as sisters had been off to a difficult start...

Mei insisted on this routine. Never bringing it up in public, but always making sure to check her when she had the chance. She wanted to make sure that her 'Big Sister' hadn't gone through another accident. Even though she knew that Yuzu still had control over her bowels. Even though she knew that it was a freak accident.

And no matter how many times that Yuzu complained, Mei kept on doing it. Which was why Yuzu had stopped complaining. Merely blushing in frustration as the black-haired girl patted her rear and complimented her for being dry...

"Of course I'm clean..." Was all that she could muster before sitting down by her desk, sighing while looking at the small tower of books that was placed in front of her. "Aren't we done yet? How much more do I need to learn to catch up?"

Mei shook her head as she put another book on top of the pile, causing Yuzu's expression to grow ever slightly more frustrated. "You're far behind. A Naughty Girl like you still needs to work hard before you can consider yourself a Good Girl, at least in regards to your academic skill."

Naughty Girl.

Yuzu trembled a little as she heard that phrase ring through her ears. She could feel her body quaking, but more importantly, she could feel something down below. Something that clashed with the strange sense of warmth and satisfaction she felt when she was called a Good Girl. Not that she would admit either of these things to Mei, because she didn't understand the effects in any capacity...

Mei was about to pick a book from the pile. Mathematics, since a test was coming up soon. But it didn't take very long for her to pick up on the faint scent of something foul, causing her to raise an eyebrow. "...Did you..."

"I-I didn't!" Yuzu cried out, as she quickly tried to pull herself off the seat. She wanted to hurry to the bathroom to deal with her emergency...

But Mei's hand on her shoulder kept her seated. "Spread your legs and pull down your panties." Her voice was firm. Hard to fight back against. That was one of the things that Yuzu had grown weaker to, especially considering their very first meeting...

The blonde girl blushed brightly. And despite not wanting to do what Mei said, Yuzu spread her legs. Even slipping her thumbs under her skirt so she could slide her soaked panties down. Thankfully, she hadn't peed a lot. But the evidence was there...

Yuzu had pissed her panties. And while she didn't understand what happened, Mei definitely did. She had noticed that Yuzu had been growing more and more receptive to her nicknames, given the lack of complaints when she called her a 'Good Girl'. But she reserved 'Naughty Girl' for accidents and associated behavior. So...

"Good Girl."

Mei said it once again, approving of Yuzu's obedient behavior. And the girl trembled once again. This time, out of a sort of pleasure. Something that Mei was able to tell given the way the girl slightly squirted down below. Indeed, it seemed as if she had conditioned her into reacting to her nicknames...

It was something that she would have to test in a much more thorough manner at a later point in time. For now, she reached for some tissues. All so she could wipe away the lingering pee on Yuzu's exposed lower lips. Carefully. Without bringing more pleasure to her than necessary.

Yuzu naturally felt even more embarrassed at the way she was being treated. What else was she supposed to feel? She was getting her pussy wiped by her younger sister. The very same younger sister that kept on calling her 'Good Girl' and 'Naughty Girl'! The very same younger sister that had accosted her and told her off when she first transferred to this school! It was terrible!

And yet... And yet..!

She'd be lying if she said she didn't feel at least a little comfort, knowing that Mei wasn't going to tease her for what she had done. If anything, Mei was very patient with her. Both when she had that accident that started their relationship, and even now. Sure, she hadn't pooped her panties, but being wiped down just because she pissed herself still felt... Reassuring.

"Remember to come to me if you need help." Mei said as she tossed the tissues away. All while walking over to grab another pair of panties for her sister. "That's what a Good Girl would do."

Yuzu trembled a little more as the blush completely consumed her face. "F-Fine. Let's just... Let's just get today over with." She muttered in defeat. Not even trying to stop Mei from helping her slip the new panties on. Right now, she just wanted to get her tutoring over with so she could go to bed and just... Forget that today had happened.

She didn't want to think about the feelings that were growing within her heart. The feelings that blossomed bit by bit after every session that Mei tutored her. The feelings that bloomed when she treated her so gently. Those feelings were overwhelming, and acknowledging that she had come to love Mei was...

No. No no. She wasn't even going to think about that. She didn't love Mei like that. Especially not after she had just wiped her down like a pissy baby. No. No no no. Not the time to think about something like that.

She just had to finish today's tutoring, and then she'd flop straight in bed. Together with... T-Together...

Mei watched Yuzu's expression shift between several stages of shame and embarrassment, before hiding a quiet smile. If this was what she was like when she had an accident at home...

Then it was time to see what she'd be like if she helped her out after a real accident...

---

"Why can't I get her out of my mind..."

Yuzu muttered to herself in the middle of class. Not at all paying attention to what was currently being taught. Completely and utterly focused on the feelings bubbling through her. Because why would she ever pay attention to something so boring when her heart was racing like a horse?

Heart thumping against her chest. Beating. Echoing through her ears. All while the images of the night before kept on replaying through her mind.

'Naughty Girl'.

Yuzu pinched her thighs together. Desperately trying to keep herself from peeing her panties again. An instinctive response to the words floating through her mind. She didn't want to end up making a mess again. Even if she still didn't quite realize that those words were why she had made a mess in the first place.

'Good Girl'.

Then the images of Mei taking care of her flooded her mind. And she felt the slightest wave of relaxation wash over her. Right before she felt an incredible flood of shame weighing down on her. She could feel every inch of her body tensing up when she thought about Mei calling her a 'Good Girl'.

It felt good. But that was why it felt awful at the same time. She shouldn't be feeling this way about a girl like her calling her a 'Good Girl'. She never thought she would come close to feeling like this...

At least Mei wasn't there to whisper it into her ear right there and then. Then she'd break in front of everyon-

"Excuse me."

The door to the classroom opened, and Yuzu could feel her body completely freezing at the sound of that voice. She didn't want to turn towards her. She didn't want to acknowledge that Mei had arrived right when she thought she was free.

But that didn't stop Mei from coming closer. She didn't look towards her, but she could feel her presence. Her daydreaming washing away, replaced with nothing but intense shame.

"My sister forgot her lunchbox." Mei said in a very factual manner, especially as she stepped over to Yuzu's desk and placed the lunch down on her desk. "And Mother called her a 'Naughty Girl' for her mistake."

While most of the class found it a little amusing that the very formal and punctual Student President would make a joke like that, even the teacher cracking a slight smile...

'Naughty Girl'

Yuzu wasn't paying attention to what the others were doing. She couldn't pay attention to anything. All she could do was let out a soft whimper. All while her posterior started to spread ever slightly...

*FRRRRrrrrrrt...*

The entire class was looking at her. Making it impossible for her to hide the fact that she had just farted. Making it utterly impossible to hide the subtle *hisssssss* sound that echoed from between her thighs. Making it improbable that anybody didn't see the state her seat was in...

Yuzu's seat was soaked in her pee. And it just got worse as the effects of the trigger hit her, causing the back of her panties to gradually push out as the wet sounds of waste hitting piss echoed out. They were all watching her. All eyes on her...

As she just shat herself right then and there, unable to do anything to stop them from looking at her. Tears running down her cheeks as she was forced to keep pushing. Forced to overfill her panties because of the shame...

Mei felt a little bad for doing this to her. Yet she had to do it. To see how far the words would push her. And they had pushed her beyond the brink, causing her to quietly clear her throat. "My sister unfortunately has both a weak bladder and weak bowels. Stress gets to her easily." She explained, trying to make it easier on her. And at the same time, make it less likely for others to blame her for this accident...

Yuzu didn't know what to say, even as Mei quietly helped her up onto her feet. Even as the poopy in her panties was threatening to overflow and force her panties off her waist. She barely even registered all of the eyes on her. All she could do was simply sniffle a little...

"I'll take her to the nurse's office. Excuse us." Mei said, bowing her head slightly before guiding her sister out of the room. Away from all those prying eyes that quickly started to whisper among themselves as they tried to figure out how a girl like Yuzu was able to handle herself in a classroom before this...

Not that she was going to let Yuzu listen to all of that. She was a Naughty Girl that couldn't keep up with her studies, yes, but they didn't know her like Mei did. Mei was going to make sure that her Good Girl remained a Good Girl. And after putting the Naughty Girl trigger to the test, she knew how far she'd have to go.

She just had to take her to the nurse's office for now, so that they could handle her accident. And begin the first step in making Yuzu a truly Good Girl, utterly dependent on the very same sister that had begun to make her fall like this...

Unaware that somebody just might be keeping an eye on them in turn, due to how much time Mei had been spending with Yuzu...

---

"You really were a Naughty Girl, filling your panties in the middle of class like that."

Mei's words hit hard, causing Yuzu to gasp as her poor bladder released its contents once again. Her panties being filled with yet more pee, making it harder and harder for them to hold onto her hips...

"I-I didn't mean to make a mess in class! I didn't mean to, I didn't mean to!" Yuzu's complaints sounded downright childish, as if she didn't know any better way to defend herself after an accident like that...

"I know." Mei said, gently kissing Yuzu on the cheek. An act that didn't help much beyond simply making Yuzu blush even more than she already did. "But we have to take precautions. You had an accident on your first day of school, and you had an accident last night. Three times is too much. Even for a Good Girl who's been doing her best to stay clean."

Hearing Mei talk down to her like that, emphasizing how she had failed, made Yuzu's heart sink further. And the way that Mei removed her panties to expose her dirty elements didn't help either, the dirty panties being discarded into the trash given the way she had ruined them with her feces and her urine...

"I-I don't know what came over me. It just happened. I..." Yuzu muttered, yet she didn't have any kind of defense. Maybe she really did have a weak bladder. Maybe her bowels really were weak. Either way, Mei entering class like that shouldn't have caused her to end up filling her panties. It was so abnormal, she didn't have any kind of excuse or answer or anything..

A sharp moan suddenly left her lips, as she felt Mei's hand brushing up against her most intimate and private parts. She knew it was because she was busy cleaning away at the mess she had made, but that didn't make it any less pleasurable. All she could do was clamp her mouth shut, even as she trembled in response to the way she was being cleaned...

Mei had to be thorough as she did this. Because there was simply too much shit that had to be wiped away. She didn't blame Yuzu for ending up this dirty, not in the slightest. She was the one that had prompted it, after all. But that didn't mean that it wasn't a chore. Thankfully, the measures she had thought of would make this a lot easier in the future.

"There's something I've been considering." Mei said, as she tossed a wipe into the trash after finishing up one of her sister's cheeks. "To make sure you remain a Good Girl and don't ruin another pair of panties."

Yuzu blinked a few times in between pleasurable shakes, before the thought dawned on her. And she very quickly started blushing even brighter than she had before. "You're not thinking of that. You can't be thinking of that. I'm not wearing one of those, I'm not!"

"I'm glad you thought of it too. Good Girl." Mei smiled, and Yuzu just trembled more in response to those words being used. "A diaper will do wonders to keep you clean. It might also keep you from getting too intimate with your books."

"N-No way, I don't deserve to wear a diaper! I can hold it, I promise! I promise it won't ever get this bad ever again! Promise!" Yuzu completely ignored the shot at her personal tastes, because the shame that filled her when Mei implied she should wear a diaper was way worse...

Unfortunately, her protests went unheard. As soon as Mei had finished cleaning her up, she helped Yuzu lay down on one of the beds in the office. And after going behind the curtain for a moment, she revealed the accursed garment. She had prepared it ahead of time. Did she know that this would happen, or was she simply prepared? There's no way that Yuzu could know.

Mei's firm grasp on her was too hard to resist. Even as she guided her through the steps of having her diaper changed. The padded garment quickly found its way underneath Yuzu's rear, before Mei parted her legs so that it would be easier for her to force the middle part of the diaper up against her most intimate areas. After that, squeezing Yuzu's legs together to keep the diaper in place allowed her to secure the wings using the tapes...

Leaving Yuzu wearing an adorably humiliating diaper. It was pink and covered in hearts. Almost as if Mei had picked out something intentionally humiliating. And given the look in Mei's eyes as Yuzu looked towards her, it had to be the truth. She was definitely mocking her..!

And yet...

"How does it feel?" Mei asked, as she gently put her hand against the crotch. Just to make sure that it was properly attached and properly sealed. Wouldn't want her sister, the Good Girl-to-be, to feel uncomfortable in her brand new diaper.

"...F-Feels soft..." Yuzu muttered as she tried to look away. But there was no way to escape this shame. No way to escape the fact that she had filled her panties three times in such a short while, and resulted in her panty privileges being pulled...

All she could do now is try and ignore the beat in her heart that grew louder and louder as Mei carefully cared for her, even if it also meant that she was being put through this humiliation...

S-She was a Good Girl, after all. She'd be out of diapers in no time...

"Good Girl." Mei said once again, as she gently kissed Yuzu on the cheek. Watching carefully with one eye as the front of the diaper grew ever slightly soaked. The diaper wasn't getting in the way of the triggers. If anything, it was amplifying them...

And she just had to keep pushing Yuzu along this path, so she could end up a Good Girl. Mei's Good Girl...

---

"Good Girl... Naughty Girl... I'll make sure to show her that she's a Naughty Girl and nothing but..!"

Outside the nurse's office, a girl watched the sisters bond. A girl that wanted Mei all to herself. Unaware of the depths of their relationship...

Himeko wasn't going to let Mei be taken away from her. She would prove that Yuzu was nothing but trouble. A Naughty Girl. All so she could be Mei's Good Girl instead...

And that book she had found would be the key...