After Shi starts listening to Hibiki, they're always cute together. The best part's when they rush into each others arms near the end of the first book.
Yutsuko who'd spent years researching how to make a human form homunculus, to replace the daughter shed lost. Once Hibiki selflessly tires to give her Shiraasan as her daughters replacement, Yutsuko touched and is finally able to accept her daughter's death and move on. But the best part? It turns out Yutsuko had sacrificed her ability to dream while she slept for her magic. Once she let go of her grief, she was finally able to dream again.
When Ahito wakes from his defeat, he finds Hibiki at his bedside. She'd stayed by his side until she was sure he was okay. She even cried over his Dark and Troubled Past.
When Hibiki offers some of her soul to help a demon, (it's not as weird as it sounds). The act in itself seems to have been enough to help.
When Shirotsuki finally speaks with Ahito. Although he can't remember his own name, he remembers that Hibiki is his close family - almost as if they were related by blood.
In the birthday chapter, when Hibiki thinks back on how she forgot her true birthday. The date she gave everyone else was simply a fake. Cue a shower of presents from her friends once the word spreads that her "birthday" was yesterday!