I really love how badly Yatsumura's death affected Aya, it may seem terrible of me, but I feel like it just proves how much Aya cared about her. Literally, she can't/doesn't speak anymore and (I think?) the limits of her stick completely changed because of it. As for this next bit, I'm pretty sure a lot of people have this theory already including myself, but I'm pretty sure that Yatsumura is the reborn Ni, who is (of course) a reborn and has the same time stop ability that she had.