Estella Mora's whole character is tragic, despite being a One-Scene Wonder. Suffering from dementia and hooked to an oxygen tank, she wastes away at a nursing home and gets no visitors, and she's totally unaware that her sons are dead. She dies peacefully at Mello's hand in a Pet the Dog moment for him.
Roger's Freudian Excuse for not liking children, leading up to his first close encounter with Wammy. Not only were his parents emotionally neglectful and traitors to their country, he was obligated to marry a girl he got pregnant. Feeling trapped in a marriage he didn't want to be in and disinclined to be a father, Roger neglected her and in private repeatedly wished that the child didn't exist. Ultimately and to his dismay, his wife died giving birth to his son, who in turn turned out stillborn. If his reaction to one of Erin's comments is anything to go by (see below), it seems he never completely recovered from this.
"I don't know if you have any kids of your own either but I don't think I need to know that to know you'd make a pretty lousy father if that's what you think."
Suddenly Roger looked stunned, like she'd just punched him in the irritable gut but was trying to maintain the stiff-upper-lip composure of an English gentleman in spite of it. Anger flashed through his eyes for a moment or two but rather than say or do anything to act on it, he regained balance of the lunch tray and passed her by to get back to Near's room. She wasn't worth losing his temper.
She shouldn't have said that.
Kimiko's troubled relationship with her parents, presumably because of her being the product of an extramarital affair, which she found out from a neighbor. She says that this, combined with her failed attempts to reconnect with her biological father, led to her alcoholism and subsequent trouble with the law. She expresses horrible guilt for not being there for Misa when their parents—with whom she never got to reconcile—were later murdered. Misa runs away right after Kimiko confesses all of this to Erin, leading to Kimiko falling Off the Wagon and accidentally killing Erin's cat.
Even though Light was responsible for his own actions, Soichiro continues to blame himself for his son becoming Kira, and since he lost his job he feels useless and that he's a failure to his colleagues, wife and daughter. Then just when you think it couldn't get any worse, his undead son comes calling to taunt and threaten him.
Misa's breakdown upon finding out that Lumen is Light, then realizing that Light never gave a damn about her and with it how empty, lonely and hopeless she truly is. Catching her at her lowest, Mello doesn't ask her what's wrong but does comfort her in his own distant way.
Erin's whole character arc. Nothing seems to be going right for her, between being betrayed by Misa again, losing her job and livelihood over a fake publication she had nothing to do with, almost being killed by a Kira fanatic for setting the record straight on TV, having her cat fatally struck by a relapsing Kimiko, and then getting stuck with an insane lovesick shinigami who is strongly implied to be L's reincarnation. She seems to be inching closer and closer to a total breakdown.
Matt having to listen to Mello and Misa having a tumble Right Through the Wall, especially since Matt is implied to have his own feelings for Mello but cannot and will not act on them. Anyone who has heard or watched someone they care about engage romantically or sexually with someone else will know how painful this is.