1. He called Ideum by his name although Ideum has not introduced himself yet
2. The line where he said he can't believe those two both know each other
I don't know if I'm reading it too much. Maybe Jiseong is the highschool classmate of Ideum. Maybe he died his hair. And it's been a long time so maybe did not recognize him.
Why I think Jiseong already knows Ideum:
1. He called Ideum by his name although Ideum has not introduced himself yet
2. The line where he said he can't believe those two both know each other
I don't know if I'm reading it too much. Maybe Jiseong is the highschool classmate of Ideum. Maybe he died his hair. And it's been a long time so maybe did not recognize him.