Let's start from the history. Minju is Yujin's childhood friends, both from Daejeon. As you can remember, Heejin told Minju that they had been friends with Hyunjin since childhood that's why they're very close, but that's part of Mr. and Mrs. An (along with Joohyun)'s plan of manipulating Minju's memories. Someone that had her memories removed would cling into reassurances from those who were with them the moment they opened their eyes.

Then we knew about Joohyun being Mr. and Mrs. Kim's foster daughter. They adopted her when they thought Mrs. Kim would never bare a child, which didn't happened since Minju was unexpectedly born.

The lack of attention and jealousy turned Joohyun hateful of her baby sister and walked out of home, planning to return with money so that her foster parents would need her more than Minju. 

That's when she found herself acquainted with Yujin. When Joohyun arrived at the household, Yujin's already devastated from Minju. 

The girl left Yujin, decision of her parents, after knowing that Yujin was actually admitted with Borderline Personality Disorder (BPD). They thought she'll be dangerous for Minju since Yujim had been in multiple fights just because of those who say mean things to Minju.

2 years after Minju left, she went into high school, became the council president and a cheerleader, and met Chaewon. Which soon became her girlfriend. Chaewon's friends became Minju's closest friends, which is our IZONE-deuls.

They got into the same courses, the medical field just in different majors, and went into the same university. Which is the start of the manipulation plan. 

Yujin went to Seoul the same year, accidentally saw Minju (probably in the park) and decided to go to the same school with the girl. Of course, with money and connections, the An's managed to keep their daughter's identity hidden.

Joohyun then placed Seulgi as the principal, Sooyoung and Seungwan as teachers. They all owe Joohyun and the An's in a way. They gave them education. 

With that, Yujin finally entered the same school on sophomore year and managed to be in Minju's classes (because the principal is the eyes and ears of her parents).

Chaewon broke up with Minju on the same year, probably sensing that Wonyoung likes her girlfriend at that time and left Minju heartbroken. 

Later on the same year, Minju's parents died. There's no details portrayed but you can say that the An's probably have something to do with it to not have any "distractions" to their plans.

Speaking of which, the An's told Yujin that they'll be away for a while and after that, they never really came back. They started building the hospital on the same place their house in Daejeon was constructed. It's a mental ward at first sight but it's just actually a cage for them to try their illegal drugs that would be tested using the admitted patients.

Yujin was one of them and Joohyun as well.

During the summer of sophomore year, Yeri came in and became a host of a summer camp. Which are all a front for their big plan. They were supposed to have the actors and the puppets. The actors were Heejin and the other members of LOONA. They were manipulated using the same tactic of getting them their degrees SURELY adter they graduated without any hussle