When M shows remorse for the first and last time, deciding to fulfill Moran's wish.
M: Forgive me, Moran.
Mary throwing away her sword and refusing to fight Charlie. They could have easily had a Downer Ending where Charlotte has given up on tomorrow and Mary has to take her down. Instead, Mary is able to talk sense into her and convince her to leave Sharanoth together.
Despite how much Mary made it clear she absolutely hated M, in the final chapter she realizes how lonely he has been, and chooses to save him.:
Mary, to M: Even if you don't want to, I won't let go. "Wait and hope", that's what you told me. Now I'm saying it to you. No matter who you are, or what pushed you into despair.
The ending, where Mary is reading letters written to her by Angie, Charlie, Elly, and finally M.