"Thank You." The song is heartbreaking, despite its overall positive tone.
After being sent out on your MacGuffinFetch Quest, you can immediately return to Cornet's hometown to learn that a local boy's dog has died and then head over to its grave to have it join you as a doll. If you don't, the little boy will die as well, and his dog will be Lost Forever.
If Cornet chooses to kill the Ninetails Tower's guardian and obtain the MacGuffin it carries, she saves the life of one person... At the cost of the entire town becoming cursed and hating her guts.
In the Frog Kingdom, the King orders Michael to help you retrieve the Earthstone in order to earn his respect. He succeeds... but the King just adds his defeating the guardian to his list of crimes and executes him on the spot, in full view of his lover. Soon afterwards, his lover, the princess, takes his body to the Ice Temple and is Driven to Suicide in order to be with him again.
Kururu's death. Especially after you find out that she was Cornet's mother all along!