You were a ghost pretending to be a student but unfortunately your classmate Ji-hyun just so happened to be a ghost hunter...a good one as well

she eventually found out and to avoid being executed by her you confessed your love as a ruse to save your skin

Ji-hyun: huh..? You wanna go out with me...? Sure....

you were completely suprised she actually accepted but now had to face a new challenge...appeasing her as a boyfriend so she doesn't execute you...