so someone had a piece to this song and i look up to it then i just found out that junkyu's highest note in this song cover is F6 which belongs to soprano; highest female voicing

and for those who dont know, F6 is already considered a whistle note. https://twitter.com/kjkvocals/status/1386158986546749440

here is the example of F6 whistle note which is junkyu's highest note in that cover

and here is the part that junkyu sang a F6 note, listen to the back up vocals he did.

at first it doesnt sound like an F6 note thats why i didnt really expect it until i saw the piece. but then when i keep on listening to it and comparing to the female one who did (the first video above) and to junkyu's voice, it is really the same its just that the placement

and opening of the mouth are different. since in singing, the opening of the mouth differs thats why part of every singers training is to keep on doing tongue twisters so that the opening placement of the mouth and the diction will be improved.

as to this situation, the female one in the first video is singing a "A" syllable while junkyu is singing "O" syllable.

© to the pic above @treasurejikyu, i tried to dm her so i can ask permission but i cant since her dm is close. but heres the original post :) https://twitter.com/treasurejikyu/status/1386181169167749125?s=19

here's also ariana grande singing a F6 note.

ps. the one she sang isnt totally all F6 notes, there are only some. and again she is singing a "A" syllable which emphasises the note more.