A lot of people are saying this is a great backstory, and while I do think it’s pretty nice, honestly I always find the whole replacement trope really weird, at least unless the author addresses it in the future. At the very least I think it would’ve been better for Noru to be very different from her first apprentice, just in personality and maybe looks, so that it would be more of her doing it properly this time instead of the feeling that she’s still trying to save and/or make up for that first apprentice.
Apologies if I’m not making that much sense, I just wanted to share my opinion. The backstory itself is pretty good, I just don’t feel like it fits into the character development very well atm. (Then again, harem manga rarely have character development...)