Captain America becomes this for his girlfriend Peggy in Agent Carter. Thought to have perished in the ocean, a major plot point of the first season has Peggy becoming a double agent to help his friend Howard Stark. Peggy even confesses that she did this just to get a second chance at keeping him safe.
Daniel's death is what prompts Sidney Bristow to become a double agent and kicks the plot in motion in Alias.
Many of Captain Santiago's errors in judgment in Alta Mar come from his grief over his late wife.
Dylan Hunt's fiancée in Andromeda, after he ends up trapped at the event horizon of a black hole for 300 years. He later tries to use Time Travel to take her with him but is told that only one person can be transported. He later finds out that she has lived a long and happy life and meets her descendants on Tarazed.
The death of Rebecca Merlyn is the ultimate cause of the events of the series. Malcolm Merlyn was never able to move on from her death, and in the process, he joined the League of Assassins, alienated his son, and devised the Undertaking to destroy the Glades in a misguided attempt to avenge her death. Robert Queen, who opposed the plan, had his boat sabotaged, causing his son Oliver Queen to wash up on the island of Lian Yu and remain stranded there for five years, allowing him to learn and hone the skills that would make him the titular "Arrow" and oppose Malcolm.
Shado's death and Oliver's role in it along with the Mirakuru driving him insane is what caused Slade Wilson to become an insane psychopath hell-bent on destroying Oliver Queen's life.
Oliver himself has had several. Along with the aforementioned Shado, he's lost Sara (twice), and Taiana. But the one that completely and utterly crushes him is Laurel, his Childhood Friend, his First Love, his Living Emotional Crutch during his five years away from home, and quite possibly the love of his life. Not only that, she was the woman his best friend Tommy loved and died for, and the immense amount of grief, regret, and failure he feels that stems from all this are what causes Oliver to regress on his Thou Shalt Not Kill rule that he adopted in the wake of Tommy's death. In Season Five, it is made very clear that Laurel's death was the one death closest to pushing him over the edge above all others, and that he can't take much more tragedy before he breaks completely like Malcolm and Slade did. In the Invasion! crossover, it's even shown that Oliver's dream is to marry Laurel.
In Season 6, Quentin theorizes that the reason why Earth-2 Laurel keeps becoming The Dragon of older men is that she lost her father (Quentin's Earth-2 counterpart) when she was a girl, so she was desperately searching for some sort of fatherly love. It's also stated that while her father's death made a dent on her emotional state, the death of Earth-2 Oliver was the breaking point for her turn to villainy. Quite fitting, since her Earth-1 counterpart's death also caused Oliver to briefly regress to his old ways. Unlike him, however, she remains single by the end of the series.
In Season 2, Iris West still mourns her deceased fiancé Eddie Thawne a year after his death in the Season 1 finale. She only moves on after Barry uses Time Travel to film a video of Eddie telling her to find happiness again.
Similarly, Caitlin Snow is deeply affected by the death of her husband Ronnie Raymond in the Season 1 finale, especially since it was not the first time he died (in her eyes). However, because of that, she has an easier time than Iris moving on from her love's death. Unfortunately, her relationship with the guy she tried to move on with did not end well either, though for entirely different reasons.
In Season 3, Barry's messing with the timeline alters the future so he and Iris never get married — he later finds out that this is because Season 3 Big Bad Savitar kills her. When Barry travels to the future in "The Once and Future Flash" to avert this tragedy, he finds out that his future self did not react well to Iris' death. He became a recluse, hidden away in S.T.A.R. Labs and continuously mourning the death of the love of his life. And he wasn't the only Barry Allen to break. Savitar is a time remnant of Future Barry who was rejected by the rest of Team Flash. Already on the brink thanks to losing Iris, this rejection caused him to go insane and try to become a god so he could no longer feel emotional pain, eventually resulting in him being driven so mad that he was willing to recreate the greatest tragedy of his life if it meant he could still exist.
Season 4 opens six months after Barry gave himself as prisoner to the speed force, and Iris has taken his final words to heart. Unfortunately, she took them too well, as in "keep running" she cuts herself off emotionally from her friends, and fully jumps into running STAR Labs in his absence. At the same time she refuses to mourn his disappearance, and can't even bring herself to sleep in the bed she shared with Barry, sleeping on the couch instead.
John Sheridan believes his wife Anna to be dead, and clearly feels pain and guilt over her loss.
Also Adira to Londo Mollari, when her murder (which he attributes mistakenly to Lord Refa) pushes him completely into the designs of Morden and the Shadows, and to a stunning revenge plot against Refa. All of this was arguably pivotal to Londo's eventual fate in the story arc.
Marcus for Susan, although she never admitted her feelings while he was alive.
Carolyn ("Ship of Tears") to Bester—not quite dead, but no one as yet knew how to bring her out of her unusual less-than-alive state either. And supposedly this was the only person Bester was capable of actually loving, by his own words. His discovery that the Shadows reduced her to that state to prep her for fitting into a battlecrab's organic systems led him to some Enemy Mine cooperation with Sheridan against the Shadows.
And per the book ''The Shadow Within'', Morden agreed to serve the Shadows when they revealed to him that his wife and daughter, lost in a transport explosion a few years earlier, were actually trapped alive in a bubble of hyperspace and suffering in perpetual isolation. They offered to release them to a merciful death in exchange for his services. We also see here and in The Passing of the Techno-Mages trilogy that the necklace Morden wears in the show was a special gift from his wife, whom he still had feelings for.
Isabelle to Galen. The Passing of the Techno-Mages trilogy reveals how Isabelle died and why Galen blames himself (he inadvertently told Elizar how to defeat Isabelle's shield).
Although we only learn of it in the novel To Dream in the City of Sorrows, Catherine Sakai is serving as this to Jeffrey Sinclair in his later appearances as head of the Rangers. She was one of the first Ranger recruits and fell into the time anomaly near Babylon 5 during a mission. Reuniting with her is one of the reasons he goes back a thousand years and becomes Valen, and the comic In Valen's Name confirms he was successful.
Better Call Saul has Matt, Stacey's husband and Mike's son. His death turned Stacey into a paranoid Broken Bird which is why Mike went back to a life of crime to provide for her and his granddaughter. She attends a grief counseling group where that helps people dealing with their lost love.
Jenny Calendar for Giles. After her death, she's frequently mentioned and seen in flashbacks and dream sequences. Though Giles has other relationships, he never really seems to get over Jenny's death. Also, at different points in the series, both Drusilla and the First Evil use Jenny's form to manipulate and torture Giles and other Buffy characters.
The spin-off series Angel arguably has a slightly twisted version in its final season. The death of Fred soon after they got together sends Wes into an alcoholic, desperate spiral. And since Illyria takes Fred's form, he's forced to still be around her every day and be driven by that constant reminder of his grief, which culminates when Wesley asks her to turn into Fred as he's dying.
Tara for Willow. Though Willow has relationships after her, most of them never quite reached the same level of seriousness and eventually ended. Even 10-15 years down the line, Tara's death remains a looming presence in her love life, to the point where Willow confesses to Andrew in the Season 10 comics that she still sometimes thinks about finding a way to bring her back.
In Charmed the sisters' grandfather Alan was this to grandmother Penny. He was killed by a warlock and the death caused Penny to fly into a blind rage and become a cold demon hunter. She also became incredibly bitter towards men, getting engaged five more times and marrying three of them.
Invoked in Community, where Annie has Abed shoot a video of herself so that in the event of anything happening to her she can more effectively fulfill this trope for someone.
Annie: You know in movies where the hero's wife or girlfriend is dead or missing and so he sits in the dark and he watches her in a home movie. Abed: Or a hologram. Annie: Or hologram, and she's always beautiful and full of love almost to the point of being stupid? We're making footage of that for me in case I get kidnapped or murdered!
Dark Hole: Gender-flipped. Hwa-sun's husband Sung-bum is killed less than ten minutes into the first episode, and her hunt for his murderer leads her to Muji.
Dark Matter: The showrunners must really like this trope, because of the seven main characters, One, Two and Three all turn out to have them. That's half the main cast. One's wife Catherine was murdered and Two and Three both have Ill Girl girlfriends - Dr Irena Shaw and Sarah, respectively - who were put in stasis. Sarah also dies, but survives as a Virtual Ghost. Played With in that they're all Amnesiac Heroes at the beginning of the series (hence the numbers as names), so each of them only later discovers that they have a Lost Lenore in the first place. And since Two and Three's original selves have been wiped, they also serve as this to Dr Shaw and Sarah, in a way, when the latter two are out of stasis.
It's implied that the Doctor might have at least one of these in his past (i.e. his unseen first wife).
Rose Tyler becomes a non-dead version of this to the Tenth Doctor when she falls into an alternate dimension, to the extent that he completely alienates his next companion Martha as he spends the next series effectively moping over Rose.
Clara Oswald becomes this to the Twelfth Doctor after she is killed by the Quantum Shade. First, he spends 4.5 billion years imprisoned and grieving her in a cruel variant of Groundhog Day, and then once he escapes that he attempts to rewrite time itself in order to prevent her death, nearly becoming a Woobie, Destroyer of Worlds in the process. Ultimately, the only way for him to move on is to erase most of his memories of her.
Grace, who dies by Heroic Sacrifice in "The Woman Who Fell To Earth", is this for her husband Graham O'Brien, who chooses to travel with the Thirteenth Doctor mostly as a respite from his grief. His decisions throughout Series 11 are strongly affected by him constantly asking himself "What would Grace do?"
Kane's lover Xana in "Dragonfire", who committed suicide rather than get captured.
Lady Sybil Branson née Crawley is deeply mourned by her husband.
In Series 4, Matthew dies and is mourned by Lady Mary; a major arc that series is getting her out of the shell she's imposed on herself since his death. It's implied that even by the end of the series, she hasn't quite gotten over his loss, but is out of her shell.
The death of William Boone's wife in the Earth: Final Conflict pilot serves to drive Boone into the role of a double agent, protecting Da'an while working for La Résistance. It doesn't take a genius to figure out that she was killed when he refused Da'an's initial offer by claiming that he wants to spend more time with his wife. While Da'an's role in Boone's wife's murder is unclear at first, Zo'or blatantly states in the Season 2 opener that Da'an was the one who ordered her death.
From Farscape, D'Argo's wife, Lo'laan. She's killed, he's framed for her murder and imprisoned. He spends much of the series trying to clear his name so he can return home and trying to find their son.
Robert Baratheon's rage and pain over the death of his betrothed Lyanna Stark still hasn't cooled after 17 years. The Season 6 finale implies that this was not really reciprocated since Lyanna was convinced that Robert would murder her son and she feared his wrath.
Robert: You want to know the horrible truth? I can't even remember what she looked like. I only know she was the one thing I ever wanted... someone took her away from me, and seven kingdoms couldn't fill the hole she left behind.
Although she is probably alive somewhere, Tyrion's first wife Tysha is still a deep source of grief for him, as Tyrion poignantly indicates whenever she comes up.
Tyrion: I was wed; or don't you remember?
Tywin adored his wife Joanna. He holds her Death by Childbirth against Tyrion, and the fact he has not remarried seems to indicate this trope.
Loras and Brienne mourn Renly after his death, with Loras eventually moving on to form a doomed relationship with the male prostitute Olyvar, while Brienne remains devoted to Renly and avenges him.
General Hospital's Sonny Corinthos has two—his late wife Lily, killed by a car bomb meant for him, and Brenda, who for years was presumed dead in a separate incident, but he blamed himself anyway. In the two decades since Lily's death, he has sabotaged nearly every other relationship he's been in because he's afraid of the woman meeting Lily's fate, and he fell for Hannah Scott because of her eerie resemblance to both women.
Turns out that while Ted was searching for his true love, the Mother was having trouble letting go of her boyfriend Max, who died in 2005 at the beginning of the series.
In the end, The Mother became this for Ted after dying from an unspecified illness. The entire point of the show, talking to his kids about the events leading up to the meet, was his dealing with the grief and covertly asking their permission to date once more.
The fact that he couldn't save his wife from a car wreck is what spurs David Banner to gamma experiments in The Incredible Hulk (1977).
In Key West, the major elements of Gumbo's backstory are based around the loss of his wife, Cee Cee, to a sudden and unexpected illness. In "The Great Beyond," Gumbo finally realizes that he has to move on with his life despite still being desperately in love with his dead wife. But that's okay, because Cee Cee still loves him, and understands.
In a variation, a Season 9 episode of Law & Order: Criminal Intent reveals that Nichols has an old girlfriend, ironically named Lenore, that he still carries a torch for. While Lenore is technically not dead, her mental condition has deteriorated so much due to schizophrenia that Nichols indicates that he feels the Lenore he loved no longer exists.
Lost: Charlotte could be this for Faraday. He is smitten with her, and then she dies. Her death makes Faraday question his entire belief system about the ability to change the past, resulting in him deciding to detonate a hydrogen bomb over a pocket of electromagnetic energy, hoping it will change things. This ultimately leads to his own death, and also forms the major narrative of the finale as Jack attempts to continue his plans.
Lost Love in Times: Feng Xian Wu died before the series starts. Her husband Yuan Ming is still in love with her and becomes increasingly villainous after her death.
Patrick's wife counts in The Mentalist. Her murder by Red John is what fuels all his actions in the series after.
Nikita's deceased fiancé, a civilian man named Daniel who was killed by Division after their romance was discovered. Daniel's murder was what led Nikita to go rogue and attempt to take down Division.
Owen has his own example in Emily.
Michael's wife and daughter are also killed with a similar effect. Realizing that it happened on the order of Division leads to his HeelFace Turn.
NUMB3RS: Margaret Eppes, who died of cancer before the start of the series, is this to Alan Eppes. He is shown to be struggling with moving on from her loss. However, he is later shown to be dating a caterer in season two and is dating Charlie's boss, Millie Finch, in season three.
Finch seems to be this with respect to his fiancée Grace—he's still alive, but because he faked his death (Or rather, made certain that nobody realized that he survived an assassination attempt aimed at the friend he was standing next to at the time), she doesn't know this.
Gender-Inverted Trope in Rejseholdet, where Ingrid's long-term boyfriend, Søren, dies from a brain hemorrhage in the sixth episode. His death continues to have lingering effects on Ingrid's private life throughout the series.
The Rise of Phoenixes: Ya Le, for Zhan Bi. She died eighteen years before the series starts but he's still in love with her. It turns out she isn't dead after all; her husband had her imprisoned in the palace and pretended she was dead... but by the time this is revealed Zhan Bi is dead.
In Season Three of the BBC series Robin Hood Robin Hood may have got another love interest in the form of Kate but the final scene of the final episode affirmed Maid Marian's status as The One True Love.
Sura in season one is almost the most classic example of this trope in a TV series since Shelly Webster in The Crow: Stairway to Heaven. Even before the titular hero knows for sure she's actually dead she appears in flashbacks and dream sequences, and after she dies in his arms. Spartacus does get another love interest, Mira but (1) she bears more than a passing resemblance to Sura, and (2) it takes him a long time to reciprocate her interest. They spend a lot of Season 2 dancing around Spartacus' ongoing love and grief for his murdered wife, and after they finally do get properly together, Mira is killed too! Word of God has it that Spartacus probably will never be able to love again.
Melitta is this both for her husband Oenemaeus and his friend Gannicus, who had been forced to have sex with her for a Roman noble's amusement (and ended up falling for her). The fact that she died while about to have sex with Gannicus complicated matters even further.
Quintus becomes this for his wife Lucretia by Vengeance. Their ludus was massacred at the end of the first season and he died, while Lucretia survived. That and the death of her unborn child turns her into The Ophelia.
Tasha Yar for Data. He keeps a hologram of her in his quarters, which becomes a plot point, and becomes friends with her sister and is hurt when she betrays him. Also, he makes an enemy of her alternate timeline half-Romulan daughter, whose appearances serve to remind him of Tasha.
Without Jennifer Sisko, the entire series wouldn't have unfolded the way it did. It led to Sisko accepting the post in the first place, him becoming the Emissary, him becoming so bound to Bajor, and his final fate at the end of the show.
A partially successful example occurs with Tora Ziyal. It succeeded via her father's storyline. He had never been entirely sane and broke completely over the death. He ended up as the Big Bad, trying to bring about a Bajoran apocalypse. She was also supposed to be this trope for Garak as his driving inspiration for every future action he took against the Dominion. However, because the show refused to openly admit the impact of her death on Kira and Garak because it wanted to redeem her murderer, the fans only learned this fact through Word of God rather than the show itself.
Mary for all the Winchesters. She's the motivation for much of the first two series and Zachariah tortures her soul ( or an artificial copy of her) because he knows it will upset Sam and Dean.
Jessica is Sam's Lost Lenore. She appears as a hallucination and in his dreams. Lucifer wears her form the first time he talks to Sam, so that he'll be more convincing ( and possibly for the sake of fanservice ).
Lucifer also tries a similar trick with his first vessel Nick, who had lost his wife in a violent crime.
And Bobby has his wife, whose death he has never quite gotten over. It's the motivation for everything he's done and if Bobby is getting an episode in the spotlight, chances are fifty-fifty that his wife will appear at some point. Supernatural loves this trope.
Inverted in a That Mitchell and Webb Look sketch involving a parody of the film Rebecca. The eponymous Rebecca arrives at her new husband's house only to find out he is obsessed with preserving everything in the house for his second wife.
Timeless: Logan's wife, who died in something he believes to be his fault. Emma resembles her, to the point that he pulls tries to her out from under the Hindenburg, despite knowing she was one of the victims of the explosion.
Male example — and doubly unique and interesting as he is the Lost Lenore to another male character — Brandon from Season One of The Wire, whose death continued to have ramifications through subsequent seasons.