Mellow: He... promised... If there's no... Jody... There's... no... me...
Chris' adoptive parents arriving at Christmas and demanding to be allowed to take him home, bringing up all the logical reasons he shouldn't be living at the shop. It just kills you, because you know they're right, and yet you also know they're dead wrong.
The entirety of Catherine's story.
The finale; D's father's 'last memory', Q-chan's true identity, the contents of D's suitcase, and worst of all, grown-up Chris meeting the new D.
Chris:I was wondering... 20 years ago, my older brother, Leon, left to return something to Count D. Something that the Count had left behind. Do you know if he ever made it?
The entirety of the Devil chapter...how you see the grandmother regress to her old days and finding out that her cute precious teddy bear was a pet her father signed a contract with, and it kept a bible and a photo of her past inside it during the beginning of World War 2. Oh god, it makes me cry right now.
This troper will raise you the spin-off story from that, dealing with Count D living in Germany while World War II is going on. Especially the scene where D and Alex stand on a statue and talk about how the humans might kill each other all off after all, leaving them to face away.
That. Fucking. Butterfly. Just his FACE first when he realizes that Chris doesn't exist, and then when he can't find the shop anymore.
For that matter, the SECOND time Leon goes to the shop and finds nothing there, and this time for real as Count D has packed up and left. His absolute refusal to accept what has happened is heart-shattering.