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Tear Jerker: The Blackwell Series
Every single one of Lauren's "I need a smoke" sequences
A lot of the ghosts and/or people you help fall into this territory. Unmarked spoilers abound below, so be warned!
From Blackwell Legacy
Alli wanted to be a vet, and her ghost was saddened because all of the dogs at the park were afraid of her.
Susan tried to commit suicide to get The Deacon to stop talking at her, losing her boyfriend and freaking out her family in the process.
All three girls had long, full lives ahead of them... and it was just a chance encounter with a ghost accidentally summoned by a Ouija board that drove 2 to commit suicide and the 3rd to attempt it.
Even The Deacon gets in on this - since he'd "got a whiff of sulfur" in the '60s (according to Professor Seagull in "Blackwell Convergence"), he'd been seeking out help from anyone that happened to talk to him... killing who knows how many victims in his desperate search for aid.
From Blackwell Unbound
The entire case involving the C-Sharps - Cecil tolerated Isaac solely for the sake of his girlfriend, Isaac's sister. After her Death By Despair (after being told she'd never sing again), Isaac refused to leave the stage without playing the duet they'd always closed a show with... but she wasn't there to play accompaniment.
Mavis Wilcox was an extreme hikikomori whose fear and habits drove off a husband and got her all-but disowned by her son, only realizing she was dead after being asked to find the one gift he'd ever given her.
From Blackwell Convergence
The first ghost of a failed businessman was depressing if only because he'd resolved to build himself back up... and then realizes that he'd died when he stepped out onto that ledge earlier.
The Countess, of all people, gets one here, as you discover that the reason she went insane was because she'd banished her spirit guide - this is made even sadder in "Deception" when you discover that she'd been tricked into it by Gavin and/or The Conspiracy he's a part of.
From Blackwell Deception
The first ghost, as much of a Jerk Ass as he is, also comes across as sympathetic when you find out that he'd stolen the money only after the bank had fired him after 30 years of working for them, and not long after his wife had died of cancer.
The entire bit with Jeremy, since his having kept the photo of him and Rosa means he did care about her - but just never got around to saying it.
All of Gavin's victims - the Haines family was torn apart by his brainwashing Penelope into disowning her family (and leaving everything to him), Tiffany was just a confused young woman trying to find a job she enjoyed, and Jamie, in the words of her professor, "She had a way of infecting you with her passion. When she loved something, she could make you love it too. But as I said, that passion faded with time. She stopped having a passion for much of anything, I think. Such a waste." Couple that with Gavin insisting she try being something she's not, and... well, the ending to that game was very satisfying to see.