When will we get a legit peek into Sister Lily's backstory? We've gotten small glimpses in her past by hearing that she was trained by Theresa Rapual and the obviously known fact that she help raise Asta and Yuno along with other orphans at the church with the priest. As of recently, she was defending the children by fighting against one of the elves and displaying a level of magical strength that cause the elf to make the remark, that her power on par with magic power of a noble. What I hope that we get to see is whether that statement is really true about the sister actually being a noble or whether she is just exceptionally gifted. In my opinion, it  most likely the latter, but I wouldn't be surprised if it was the former. Given that there are elves in the town of Nairn as well, we might just see Lily and Theresa reunite and see a flashback of the training that she went through. 