kids on parallel lines

Ji-heon (female, 17), runs down the hallway every morning because she doesn't want to share the elevator with Hyun-min (male, 18), who lives upstairs in the same apartment. The one who doesn't want to face Hyun-min is Hyun-min's father, the mother's lover. They seem to be trying to take their mother away from Ji-heon. Then, Ji-heon witnesses his mother's attendance as Hyun-min's mother during the parent interview at the school.