To unlock Ji-Min, you need to beat the game on hard mode and achieve the ending White Crysanthemum or Ivy. When you start your next playthrough, you will meet her near classroom 2-8. Then after defeating the demonic tree, but before picking up the wood token, you need to find 10 doodles throughout Main Building #1. Once all doodles have been found, pick up the wood token. After a few scnees investigate the noise and enter the northern staircase. Another scene with Ji-Min will trigger, if all doodles have been found. Finally, in Main Building #2, you need to check the security footage inside the principal's office three times. You will see Ji-Min entering the biology lab's backroom. Go there and a timed mission will start. After solving this mission, you're playing as Ji-Min and her scenario is unlocked for this playthrough.