Comment by Yoman Aku on Zensei wa Ken Mikado. Konjou Kuzu Ouji - Chapter 26

I think the reason she is not telling him is because she knows he will start relying on her, in an emotional way. Fai is a person who is dependant on other people, as in looking someone to protect or finding the reason to live from other people. If she tells him...I think he will not change for the better or not continue to look for his own reason to live Ó⁠╭⁠╮⁠Ò