Nitori -> Aya, probably through Hatate telling Nitori about her. Aya -> Hatate and Hatate -> Aya. Then Aya <-> Momiji and Hatate <-> Nitori. Strangely, there's nothing between Hatate and Momiji, despite what's in the comics. Since I can't read Japanese I can't tell you what any of the notes actually say, though.