Mina's role in the first game is both minor and major, but more directly, minor. She is mentioned several times throughout the game as being Youngho's friend and neighbor. They usually walk to school together and it's made clear that Myung-Gil Kang has a crush on her and wants Youngho to keep away from her.
In the first game, her shade is shown to be severely injured and asked Youngho for help. After returning to heal her, the shade vanishes while thanking him. Once he meets Myung-Gil Kang's shade, Mina's shade comes back and tells Myung-Gil Kang off for treating Youngho so poorly, thus making Myung-Gil's shade disappear and increasing Youngho's grade in Social Studies.
You only get to truly see Mina in the first game during the ending. After Youngho passes into a coma, she rushes to his ambulance in panic and gains the pendant that Youngho was holding. Myung-Kil bad mouths Youngho and Mina tells him off. While not shown, it's stated that Mina would check up on Youngho every few days for the next few weeks.
Unlike the first game, Mina's role is very major. She is the protagonist and hero of this story. Mina has continued her daily life, while also wondering what will become of her close friend, Youngho (The protagonist of the first game). She would visit the hospital every few days and even have her best friend Da-Hyun accompany her.
Mina's adventure starts in "Bulldog's" classroom (The school's very strict math teacher). After leaving class, she's messaged by Da-Hyun to meet her in the bathroom. She tells Mina that Myung-Kil (The school bully/jock) wrote a letter for her, asking her out much to Mina's disgust. Da-Hyun then states that she's tired of dealing with him too. She then asks Mina to buy her a candy bar.
Mina goes to the cafeteria and buys a candy bar for Da-Hyun and as she does, Da-Hyun rushes in and tells her that Myung-Kil is heading her way. Mina asks Da-Hyun to stall him, much to Da-Hyun's displeasure. Myung-Kil comes in and starts off with his usual insults by calling Da-Hyun "rash girl". Da-Hyun gets mad at him and asks what he wants. He asks if she passed on the letter to which she responds that she didn't yet and has not seen Mina. Across the cafeteria, Seho watches as this all goes down and notices Mina hiding behind the water tank. Myung-Kil then insults Seho by calling him "Sehoser" and asks him to fetch Pizza Bread. At this point, Da-Hyun has had enough and tells Mina to get out of hiding. The two then run out of the cafeteria, embarrassed.
She then goes to her next class taught by Ms. Song. She meets Mr. Chance who claims to be an English teacher and is asked by Ms. Song of Yungho's state with genuine concern. Mina says she's been visiting him every few days and Ms. Song gives her encouragement. Mina takes her seat and class proceeds. She gets a B+ as her grade and leaves class knowing she could've done better. While she was about to leave school, she notices strange activity and goes down the school stairwell. There, she spots Seho standing in front of a bright door in a dark hallway. He enters and she follows. After going deeper into the room, she sees a blood moon out the window and reads a note. The note states that it would allow "the vicious sisters" to walk their world and they would release someone's damned soul. (*SPOILER* This note was very likely written by Seho.)
Not Long after, Mina passes out and enters the Coma thanks to her possession of the pendant and a trap set up by Seho. The pendant is taken off her person as the thief (Seho) runs off. She is spotted by several unfamiliar faces while unconscious and left there to wake up alone.