This is a

Jessica + Key = KeySica

Onew + Tiffany = OnFany

Yoona + Taemin = TaemYoon

Yuri + Minho = MinYul

Seohyun + Jonghyun = JongSeo

But this story mainly about KeySica and OnFany

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Jessica and Tiffany are sisters, the mother of Jessica and the father of Tiffany got
married so Tiffany Hwang and Jessica Jung are stepsister but the love each other so much

and they have a room to share but the room is really big

3 times as big as this

Key and Onew were childhood friends since the day that Onew dropped gravy on Key's head

they were best friends ever since then