Mukuro had been told to reunite with her sister, Junko, only the week before. She'd been contact by her, saying she'd developed a plan and need her assistance. Always being one to follow her sister, she'd made plans to return to Japan. It's taken a week to pull away from her work, but she'd finally made it.
Looking around the airport, she failed to notice her little sister. As she walked toward the exit, her phone rang. Despite the unknown number, Mukuro answered the call.
"OMG, I bet you were extremely disappointed at how your sister reached out to you and when you arrived, she was nowhere to be seen. I wish I could see the despair in your face. Actually, wait, then I'd be there and you wouldn't…never mind." The voice on the phone was undeniably her sister. Her spastic way of speaking flowed onward. "Anyhow, I'll send you the address to meet me at. And just for fun, you have to walk all the way here. Ta ta!"
Beep. With that tone, the phone hung up with Mukuro unable to get a word in edgewise. Suddenly a mail flashed on screen showing an address on the other side of town.
Mukuro hefted her bag on her shoulder and exited the door. As she began walking, dark clouds gathered overhead. The sky decided to pour all their contents upon the city and the Ultimate Soldier.
"No doubt she knew this was going to happen. She wanted me to trek all over town in the pouring rain. That's just like her." Mukuro thought to herself; a slight blush rising on her freckled face.

An hour of walking in the rain later, she arrived at the location her sister sent her. The building seemed to be an old factory. She stepped inside knowing Junko was the type of person who'd plan a meeting at a place like this.
"Hello, sis, I'm here like you asked!" Mukuro shouted out unaware of her sister's whereabouts in the place. "I hope you don't mind if I change."
As she moved off to the side near the wall to slip into a change of clothes, a shadow began to move. Just as she finished putting in her school uniform (the first change of clothes she grabbed), the shadow spoke.
"Long time no see, you haven't changed at all."
Mukuro turned towards the sound and stood in awe at the sight before her. The appearance and dress made it obvious it was her younger sister, but the shape of the person was different from before. Once a small, busty figure had completely been reworked into a far bigger one. The girl sported a huge stomach, chunky thighs, massive breasts, a second chin, and chubby cheeks.
"Junko…?" Mukuro questioned in disbelief.
"You mean you don't recognize your own sister? Boo hoo, boo hoo hoo." Junko said as she started to cry obviously fake tears. Almost immediately, she flipped 180. "Yup, that's right, your little sister transformed into your bigger sister. Packed on around 170 pounds since we last met. Now you see about my plan. If I'm going to have you impersonate me, you're going to look more like me."
"What…" Mukuro stammered, still trying to get a read on the situation. "Can you please expla-"
Mukuro was cut off by a poking on her leg. She looked down, seeing a little black and white bear. The bear motioned her to bend down and as she did, 4 sets of mechanical restraints grasped into her limbs.
"What's this. Junko, can you explain what's going on." she begged.
"Geez, I already told you. Guess it's just that stupid, thick skull of your. I'm going to make you look like your little sister."
Two more Monokuma rolled out a machine attached to a huge tank. The first bear that had poked Mukuro pulled an extendable tube from the machine and approached the girl. Junko strolled over to her captive sister and grabbed her by her chin.
"Please, Mr. Monokuma, I'm soooo hungry. Let me have something to eat." Junko mockingly voiced the words to her making her sister talk. After the last statement, she left her sister's mouth open and the Monokuma jumped up to put the tube into it.
Mukuro tried to speak, but the tube latched itself around her head, making speak impossible. She saw the third bear flip a switch and something proceeded up toward her. Suddenly a sweet flavor rose in her mouth and down her throat. The taste of vanilla washed over her and gave her slight sense of enjoyment. Unable to resist the flavor, she began to assist and swallowing the cream that flowed.
Junko watched as her sister continued to gulp down the concoction. She'd convinced several other schoolmates to help her develop the icing under the impression it was for a celebration she was planning. She'd tried some of it and before she knew it, she was the way she was now.
Mukuro continued gulping down the sweet cream that flowed unaware of the effects. Her once trim stomach began to swell outward. The dome rose higher and higher as she continued. Eventually she noticed a slight discomfort, but the sweetness stopped her from questioning it. The discomfort came from her stomach reaching its peak of being contained in her shirt and the buttons began to strain.
Pop!
Pop!
Pop pop pop!
Each button flew off as her stomach settled from it confines. Elsewhere another discomfort began. Her fatten rear and thighs bulked up raised her constricting skirt. Pudge formed all over the former skinny girl.
Mukuro was experiencing a great joy at the taste when something snapped her back to reality. The flow of vanilla stopped.
"Darn, we're out." Junko said with a sigh. "And that's why I've got more!" She spun around as the Monokumas loaded another tank to the machine.
The raven-haired girl was taken back into euphoria before she could register the changes her body took. Her body was around the same size as her sister, but lacking in the feminine aspects as normal.
"Mmmmm" she managed to moan out from the taste flooding back into her.
The growth process continued with no signs of stopping. What once was to make her big like her sister had gone off the rails like many other plans made by the sister. Junko just wanted to watch her bigger sister be just that: bigger.
Tears began to appear in Mukuro's black knee socks, bits of flab oozing out. Eventually her skirt gave up the ghost and with a loud RRIIPP became two pieces that floated to the ground. Her globular stomach bulged bigger and bigger. 300, 400, 500, the numbers kept getting higher and higher. When the tank need a refill, the Monokuma placed a new one in it. Throughout this process, Mukuro remained blissfully unaware of her surroundings and the taste never tired because of the components it was made of. Her white shirt came off. Clad in only underwear, she continued to get bigger. Junko noticed the supply was getting towards the end, but didn't care. Her sister was massive.
SNAP!
SNAP!
The loud sounds were evidence that her underwear could no longer keep up with the growing girl.
The (at this point former) Ultimate Soldier sucked more down, but was soon disappointed. The flow had stopped with refills no longer available. She no longer cared about what happened to her body, she was content with everything around. She was free to go (the restraints have let go a while ago) if she could, but she couldn't. Her mobility was long since gone.
"Ooph!" she let out a slight cry as a heavy object hit her. She looked down and realized what it was. She'd growth absolutely massive, a size she never knew a person could ever reach. The object that had hit her was, in fact, her younger sister jumping onto her gut. It amazes her how her sister, a five and a half foot girl, could fit her entire body onto her gigantic sea of a belly.
"Ya know, I never knew I'd have The Blob as a sibling." Junko joked. "I wonder if I could rent you out as a bounce castle? Or may…" she gripped a big portion of her sister, "I could use you as a giant bed. I mean, you're soooo soft."
"Um, sis, how are you going to-" Mukuro started to say before being cut off for the umpteenth time that day.
"Are you going to say, get me back to normal? Why would I do that to my at least one ton sister. Can you imagine how much work I'll have to do just to take care of you. I'll never have time to live my life. My life wasted on taking care of my fatty, fat fatty of sister, such despair."
The rotund, 2000 lbs. plus freckled girl thought about her life now. She'd be stuck here, in this dingy factory, forever an immobile blob. Her sister would take care of her and play with her as she pleased.
"Well," she thought blushing, "as long as Junko is happy, I will be too."