You & Seulgi have feelings for each other and it's obvious. But she's also too nice. Nice enough to let Krystal her best friend try to pursue you again. 

You & Krystal never worked out. If she was down then you were up. If she was left then you were right. You both couldn't see each other's perspectives. 

goodbye friend welcome the new title girlfriend,

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      To Seulgi you were her weakness. But there was also another thing that made her weak. She was too nice. When Krystal tells her about the crush she has on you she doesn't fight Krystal on it. Instead, she steps back from pursuing you and allows Krystal to make her advances towards you. That's what made Irene and the others upset. 

" Seulgi are you seriously believing Krystal's words? " Irene snickers

" Krystal doesn't lie though. " Seulgi replies

" And she's also the type that doesn't develop crushes on people out of nowhere so what is benefitting her when she dates your crush? " Wendy asks

The more Krystal's name was mentioned Seulgi noticed Kai's bitter expression. 

" What's with him? " Seulgi asks

" He's bitter because your little crush stole his. " Wendy remarks. 

" But didn't she already mention that Krystal's not her type? " Joy asks 

The group remembers asking you about your ideal type and you say that you want to date someone that clicks well with you.