You are a ghost that has been haunting the PK Academy for a while now by playing instruments after dark I the music room

Saiki's 'friend' Toritsuka heard these rumors about it and forced Saiki to go with him after school

As soon as it hit after dark, Toritsuka and Saiki went in and soon heard you playing music in the music hall, so they went over

Saiki took off his transparent glove and put a hand on Toritsuka's shoulder so he could have access to his powers so he could see you