Well he's right about one thing: It is Ideum's choice.
But who would trust someone who goes about things the way Jiseong does?
Lurking in the locker room waiting for Ideum, saying things about Jaeyi that don't seem to match up to reality and going on Jaeyi and Ideum's relationship in such a smug way when it's clearly making Ideum uncomfortable. It doesn't seem like a great start.
Sidenote, notice how different they brothers reactions are to Ideum's scar? Jiseong offers Ideum cover-up tape for it, whereas Jaeyi (while forceful about making Ideum show it to him), goes out of his way to kiss the scar and show that not only is it not a bad or gross thing to have, but rather something to be thankful to since it allows Ideum to be healthy enough to experience all sorts of things.
I miss her POV :') Don't get me wrong this is great too though!