Jungkook explained everything to (Y/N) - how he asked Leanne something, her suggesting it'd be better to ask Jimin, himself then ringing Jimin and finding another girl on the other end of the line. Not to mention, Jimin also calling this girl his 'jagi'.

(Y/N) could see how disappointed and sad Jungkook was; who wouldn't be?
(Y/N) herself felt a mix of emotions. She felt angry at Jimin, sympathetic at Jungkook and worried at Leanne. Every since the sleepover, her and Leanne had become closer, almost acting like real sisters. 

They both came to an agreement to subtly warn Leanne about Jimin, and when he was there, they'd both ignore him.

~

They both walked back to BTS' dorm, knowing the rest of the guys would be back, apart from Jimin. He was probably going to spend his time at his 'jagi's' house doing hell knows what. (Y/N) also invited Rosé, just for extra advice from another female about the situation. Leanne had informed Jungkook that she'd be going to one of her friends house after finishing her essay, which meant she wouldn't be there when tey discuss the matter.

~

"HE DID WHAT!?!"

The guys and Rosé couldn't believe the news they were just told. Jin brought his hands to his face trying to cover up his disappointed look, Suga started showing emotion compared to usual Motionless-Min, JHope had his familiar and friendly smile blown off of his face, Taehyung looked as if he was about to cry and Rosé look prepared to slay a . They all never expected Jimin to do such a thing - let alone cheat on Jungkook's sister and their best friend. They felt betrayed, disappointed, failed. They expected so much better from him.

"Kookie, do you know her name?" Namjoon asked, trying to calm the already-tense mood.

"Ani, but I have an idea of who it could be..." the rest looked at Jungkook with a questioning face, before he asked them the following question:

"What's the name of that girl who always hits on Jimin when we're in the canteen?"

That got everyone thinking. Lately, one of their so-called 'fangirls' kept bothering the friendship group, specifically Jimin. He never really took notice in her when she was around, but recently the boys would spot him flirting with her, despite dating Leanne. However, they thought it was nothing more than him teasing them. Coincidentally, she had a little sister who was in the same year as Leanne. She was the queenka of Leanne's year, and Leanne would always complain about how she was so snobby and y towards her and her friends: Leanne also mentioned how she'd bully her due to orders from her older sister when they found out Leanne was dating Jimin. But when Jungkook ever did anything about it, Leanne would stop him, saying it's not worth it: she was too forgiving, and would probably take a bullet for him - even after finding out what he did. 

~

Leanne came home after going to her friends' house, and was greeted by everyone sat in the living room. Everyone apart from her beloved boyfriend, Park Jimin.

"What's happening?" she asked out of curiosity, since she knew everyone only gathered in the living room if something serious was happening.

"Oh nothing..." Jin said, trying to hide the sadness in his face.

"Have you seen Jimin?" Leanne asked. Even just at the mention of his name, everyone's face darkened.

"Yah, what happened?" She asked again, now seriously suspecting something was actually wrong.

And almost as perfect timing, the one and only, Park Jimin entered the house.

"Oh, and he finally turns up!" Suga sarcastically said, before giving a last glare at the male.

"Leanne, I've decided you should go and live with Mom and Dad for the time being, I don't want you getting hurt" Jungkook told his younger sister.

"Yah Oppa! You can't do that! Not out of the blue, like that. They live in Busan - that's miles away from the University!" She exclaimed.

It was true, the university was miles away from their family home. But, Jungkook wanted to make sure she stayed as faraway as possible from her tard of a boyfriend. Even if it meant he'd have to pay for her train journey every day: anything is better than seeing his sister suffering obliviously.

"Jagi, what's happening?" Jimin asked Leanne, seeming 'innocent' to the situation.

"Jagi my ! You're what's up," (y/n) said, answering his question, walking up to him during the process. She was already pissed at how he was trying to act clueless, and decided to talk to him instead of Rosé, since if she did, things would turn ugly from 0 to 100.

"You're the reason why Leanne is having to move away, you're the reason for splitting up the siblings, you're the reason for making our friendship crumble" she said, pushing him with more force at each phrase. "Please, do not consider us as 'friends' she spat, sitting back on the couch as if nothing happened.

~

Rosé went up to Leanne and lightly tugged at her wrist, bringing her to her room.
"What's happening?" Leanne asked again, hoping at least Rosé could answer her question.
"Leanne-ah, it's better if I don't tell you and you just pack your things..." Rosé said, leaning against the door frame.
"Did Jimin do something?" 
Rosé casually went inside and sat on Leanne's bed, a sigh escaping her lips.
"Let's just say, if i told you what he did, you'd break down and feel hurt, so, we're all trying to help you get away from your of a boyfriend."

Leanne remained silent as Rosé talked. What had Jimin done that made them hate him so much? They were fine a couple of days ago. 
She began packing her clothes and school equipment, since she still had things at her parents house. 

(Y/N) came upstairs to see how Leanne and Rosé were doing, and decided to help Leanne pack.

~

Meanwhile, the boys were still downstairs in the living room, the air tense.
"We'll give you 5 seconds to admit your wrong doings. Now, the outcome could change in these 5 seconds. You could try and prove you're innocent, which I doubt you are, or you could confess you're true intentions of dating Leanne." Yoongi stood his ground when explaining to Jimin. No expression, no emotion, nothing. 

"5..."

Silence

"4..."

Silence

"3..."

Silence

"2...-"

"OK, I did, kind of, cheat on Leanne." Jimin said. And that didn't help dampen the tension in the room.
"Get out," Jungkook said "or stay inside your room forever. I don't want to see your filthy face. I can't believe I actually respected you as my Hyung."
"You're not going to tell Leanne, right?" His voice sounded, worried almost, which was, well, pathetic. 

"I don't think we'll need to tell her" (Y/N) said walking down the stairs, with a shocked Leanne following her.
"She heard everything."