Our hero is in a duel with a crafty Shape Shifter villain. It looks like the shape shifter is going to lose, until he busts out the most underhanded trick of all — suddenly impersonating one of the hero's loved ones or True Companions, trying to cause him to lose his will to fight.
Usually, the hero ends up struggling to find the willpower to continue beating up someone who looks just like his mom/girlfriend/little sister. Bonus points if the form the shape shifter takes on is that of a Dead Little Sister, for extra guilt.
Also covers clones and other doppelgangers who suddenly reveal that they happen to resemble the hero's mom/girlfriend/Dead Little Sister/Dead Pet/whatever.
If the protagonist is an Anti-Hero, there's the possibility that they won't care at all about what face they are kicking in as long as they know whose face it actually is - the bad guy's.
There's also an increasingly common subversion: because of personal issues between the hero and the character the shape shifter takes the form of, the hero actually becomes more willing to attack. Also, all the shape shifter may do is succeed in pushing the Relative Button.
Also see Shapeshifter Swan Song.
Envy positively loves this. In a pivotal moment of both anime and manga, he uses this to kill Hughes by transforming into a replica of the wife he adores, which makes poor old Hughes hesitate just long enough to get shot. And given Envy's regenerative powers, it wasn't really necessary either, it was just For the Evulz.
During his climactic fight with Ed at the end of the first anime, Envy cycles through the forms of everyone who's been either hurt or killed as result of Ed's actions. He tries to psyche him out by turning into Dr. Marcoh, his mom/Sloth, Hughes, and Mustang (which only makes Ed attack him harder). Ed keeps his cool through the whole thing, and even goads Envy into showing his original (human) form. Let's just say it was one family reunion Ed was not expecting... Ed actually invoked the use of his mom by saying he still killed Sloth when she tried it.
Envy also does this in its fight near the end of the manga: Mustang finally gets to confront Envy about who murdered Hughes. Envy responds by shape-shifting into Hughes' wife, and laughing at how easily he was tricked. Mustang is not amused. During the fight, they end up in some narrow corridors. Envy appears in Hughes shape and makes Mustang momentarily flinch, but he then incinerates Envy as he knew Hughes was dead. Envy flees again.
This trick was used a couple of times in the first anime. Lust did it to Scar, on accident, as his brother had made her to resurrect the woman they both loved. Wrath did it both on accident and on purpose to Izumi. Sloth also did this to Hoenheim in their only meeting, the only time that this led to what would normally kill someone.
In One Piece, Mr. 2 Bon Clay uses this as a desperation attack against Sanji. Sanji heartlessly beats up all of the doppelgangers of the male Straw Hat pirates, and Bon Clay is about to give up till he carelessly switches to Nami, which paralyzes Sanji with lust. Bon Clay comments, "It can't really be that easy, can it?"
Unfortunately, this turns out to be a double-edged sword, because he can't perform his more powerful attacks and maintain his disguise at the same time, due to his own body being pretty much the only one that can perform them.
The fight becomes somewhat comical once Sanji realizes Mr2's power is canceled if he touches his face with his left hand, and tries to trick him into doing so...
In Saint Seiya Lyumnades Kaza did this thrice on-screen (plus a couple off-screen times). First in a battle against Cygnus Hyoga, taking the form of Aquarius. Then against Pegasus. Then in battle against Shun by transforming into his brother; it didn't work at first. He did it again when Shun was getting the upper hand, and it made Shun doubt enough for Kaza to beat him. Finally he did it against Shun's brother Ikki by transforming into Shun. But as Ikki was a Genre SavvyAnti-Hero, it didn't work.
Better yet, Ikki beats Kaza by transforming into him, and Kaza whines that he's unable to hit his own image. This shows how cowardly he is,because this wouldn't have worked on any of the heroes.
In Bleach, the hollow Grand Fisher has a lure on his head. By reading his target's psyche, he can transform that lure into the person the target cares about the most. He impersonates Ichigo's dead mother early on in the series.
Later on, an Arrancar also impersonates Kaien Shiba, Rukia's mentor, to great effect. This particular one happened to be the 9th Espada, Aaroniero Arruruerie. In both cases, the impersonator had eaten the impersonate-ee.
The Sorcerer Hunters manga had a variation: We eventually learn that Body Surfing demon Almond Rassel had been trapped in the body of Apricot, the Missing Mom of Carrot and Marron. In the final battle, Almond has to fight Onion Glace (Carrot's Crouching Moron HiddenRetired Badass father), and plays his current body for all it's worth. Onion has no problem attacking Almond's clones, but can't bring himself to attack the real body of his wife. His solution: grab on to Almond and stab them both with his boomerangingBig Freaking Axe, which he aimed at himself.
Parodied in Ranma ½, when the Cat Demon possessed Genma, thinking that Ranma wouldn't attack him. Ranma's response: IT'S EXORCISM TIME!
To explain further, it really didn't help that Genma forced Ramna to fight a giant cat since, no matter how weak a demon it was, Ranma is utterly terrified of cats. Right before booting his son towards said terror, Genma says that it's a martial artist's job to defeat demons, no matter what. Ranma repeats that right before beating up his old man.
Dance in the Vampire Bund: One of the assassins sent after Akira in vol 4 pulled this. It worked perfectly when he took the form of Graham, a colleague whom Akira killed under as yet unknown circumstances, but then she(?) twisted the knife and tried to extract information by taking the form of Mina. Big. Mistake.
In Project ARMS, one of the cyborgs that Hayato fights tries to trick him by impersonating Ryo. In this case, it was done to trick Hayato and not guilt him. After Hayato figures out the truth and impales him, the cyborg takes the form of Hayato's dead mother before dying.
Dougan in the Saiyuki Requiem Movie absolutely adores doing this; so much so that it becomes pretty much the premise of the entire movie. For Sanzo he changes into his mentor Koumyou as well as his teammates Sha Gojyo, and Son Goku. For Goku he changes into Genjo Sanzo. For Sha Gojyo he changes into Hakkai and induces stabby death onto him. And for Cho Hakkai he shapeshifts into both his dear sister/lover Kanan as well as his friend Goku.
In Neil Gaiman's The Sandman, Loki attempts to pull this on the second Corinthian, cycling through a series of forms (starting with Dream, followed by a dragon, the Corinthian himself, the toddler who the Corinthian is supposed to be rescuing, a flame, and finally his true form) trying to find one the Corinthian won't strangle. The Corinthian, needless to say, is not impressed.
The Elementals by Bill Willingham did an elaborate variant which revealed that a Shape Shifter that the Elementals had fought before had, over the course of a Story Arc gotten one of the team, Morningstar, to fall in love with him/her.
Rare heroic example comes from Wild C.A.T.S.. When team was fighting Lord Entropy, impossibly powerful madman who wanted to take a revenge on Lord Emp from killing his wife, Voodo used her illusion to look like her. Entropy was so confused, that Emp has enough time for preparation to whooping the floor with him.
Another heroic example in Runaways, when Xavin shifts into the late wife of a man-turned-monster to talk him down from his destructive rampage.
Astonishing X-Men: Subverted (and inverted?) in Joss Whedon's run. Kitty Pryde is confronted with Colossus (the genuine article); not believing it's really Colossus, back from the dead, she tells him that if he's a robot or a clone, she'll understand, but if he's a shapeshifter or illusionist, he's in for a new meaning of pain.
Fan Fiction
In the Kingdom Hearts fanfic, A Match made in Hell, the Big Bad tried this separately on Sora, Roxas, and Pete, transforming into everyone they knew, including Organization XIII.
Film
In the Spiderwick books and Film of the Book, Mulgarath attempts to do this to the main character by transforming into his dad. Much like in the FMA example, the main character has Parental Issues and this ends up being a bad choice.
In the Mortal Kombat: The Movie movie, the villain Shang Tsung tries to pull this gambit with Liu Kang by assuming the form of his dead brother Chan, whom Shang Tsung had killed at the start of the movie. It doesn't work.
The monster in Phantoms turns itself into a little boy who looks like the kid Ben Affleck's character accidentally killed in his former life in the FBI.
The T-1000 from Terminator 2 made use of this trope, shape shifting into Sarah Connor in an attempt to lure and terminate her son John. The attempt fails when the real Sarah appears behind the T-1000, and John trusts her characteristic approach ("Get out of the way, John") over the duplicate's attitude of slumped defeat. The film's special edition, disappointingly, replaces this subtle indicator with an obvious shape shifting flaw (warped and misshapen feet).
Inverted in Dreamscape. Assassin Tommy Ray murdered his father as a teenager. While fighting him in a dream, the heroic Alex distracts him by changing his appearance to that of the father and asking Tommy Ray why he killed him.
In The Exorcist, the demon takes on the voice of Father Karras' recently deceased elderly mother.
In Event Horizon, the title ship has taken over Dr. Weir. In an effort to get to Captain Miller, Weir takes the form of a flaming Edward Corrick, the man in Miller's back story whom he had to leave behind in his greatest failure. Miller doesn't buy the attempt to guilt-trip him for a minute though: "No. You're not Edward Corrick. I watched him die."
Mystique tries this on her captors in X-Men: The Last Stand. She transforms into the senator she has been impersonating and threatens to have them all court martialed if they do not release her, and then a little girl who begs and cries to be let out. She finally turns mockingly back into herself when one of the guards threatens to empty a can of pepper spray in her face if she doesn't knock it off.
Literature
Older Than Print: In Arabian Nights, one story involves a hero being told that he must kill a demon that is taking the shape of his mother. He refuses, is sent back where he started, and has to undertake all the other challenges of the quest again, as well as kill the copy of his mother.
In the novel Myth-Taken Identity, a group of shape shifters are running around disguised as the protagonist of the series, and their chief uses it against the protagonist's mentor in the climactic fight.
A very neat example takes place in Roger Zelazny's Lord of Light during, well, calling it fight would charitable, between Yama, the God of Death and Mara, the God of Trickery. The first of many Crowning Moment of Awesome for Yama.
The Star Wars Expanded Universe plays this one straight in Sacrifice. Jacen disguises himself as Ben when dueling Mara.
Magic Time: There's a whole swarm of these towards the end of the final book, divided into groups targeting each main character. (For instance, a doctor who survived Chernobyl fights animated radiation corpses. What, the title made you think it was a kids' book?)
In Juliet Marillier's Child of the Prophecy, the Evil Matriarch Oonagh does this during the middle of the climactic showdown: while facing off her son and granddaughter, she takes on the appearance of the son's dead wife. Made worse by the fact that she killed the wife in the first place. This throws her son off-balance long enough that she would have killed him, had his half-brother not intervened.
A spell in Harry Potter and the Deathly Hallows gives Ron Weasley a double-dosage of this trope - the two people closest and dearest to the victim manifest and give thorough counterpoint arguments for why You Suck.
A Boggart does this to Mrs. Weasley in Order of the Phoenix, tormenting her by transforming into an image of her children, dead. Comes true in Deathly Hallows.
In Perelandra, Ransom is fighting the Un-man (the demon-possessed undead corpse of Weston, the previous book's villain) when it suddenly reverts to Weston's actual personality and begs for mercy. Ransom ignores it, and the narration points out that Weston's actual soul had most likely been completely subsumed long before.
In a non-villainous example (although Your Mileage May Vary, considering the character), Bartimaeus of The Bartimaeus Trilogy regularly appears in the form of Kitty Jones in the beginning of Ptolemy's Gate, purely because he knows how much guilt his master still feels over Kitty's apparent death in the previous book. He sometimes exaggerates the curves, though.
Live Action TV
An episode of Red Dwarf features a genetic mutant which uses its shape shifting ability to drive its victims to extremes of some emotion, which it then feeds on/sucks out. So at one point Kryton becomes the victim of a literal Shape shifter Guilt Trip.
The Supernatural Season 1 episode "Skin" does this when Sam fights a shape shifter disguised as Dean. Sam can't kill him, so Dean does.
Averted in Episode 5.05 "Fallen Idols" in which a pagan god is about to take the form of John Winchester and gets killed before that happens.
It happens again in Season six. When the boys prove unwilling to listen to her, The Mother Of All takes on the form of none other than Mary Winchester. Still doesn't stop the Winchesters from ganking her.
Subverted in an episode of Angel, when the Monster of the Week appears as Wesley's distant father and is unceremoniously shot. Notably this only happens after he threatens Wesley's love interest, and since Wesley did think it was his real father at the time he spends the next episode absent, apparently dealing with the revelation that he would have been willing to kill his own father. Made more humorous with all the other characters saying how they killed "their" fathers or mothers and haven't looked back since.
Although not done intentionally, the demon Illyria who has "hollowed out" and taken over the body of Fred notes that Spike has no problem punching her in the face.
Illyria: This shell... you had affection for it.
Spike: Tons. Loved the bird.
Illyria: Yet you strike at her form without sentiment.
Spike: You ain't her. I can see it. Lord knows I can smell it. And I got no problem hitting it.
Also done in the Doctor Who story "Planet of Fire".
On Heroes, Candace (who can actually cast illusions, but used it in a way equivalent to shapeshifting) as Simone to Isaac.
Notably averted on Alias. The heroes want to kill her extra.
The Worm creatures in Kamen Rider Kabuto occasionally do this by showing the image of the person they'd replaced. One early episode has a Worm guilt tripping Kagami with the form of his brother, noting that since the Worm has all the memories of his victim, if the Worm dies the victim will be killed "a second time".
It happens again in Kamen Rider Double's movie, Begins Night, in which a dopant poses as Shotaro's mentor and in turn, Kamen Rider Skull. When it is revealed that it's the Dummy Dopant doing the imitating, this pisses off Shotaro.
Lost: The Smoke Monster aka The Man in Black did this a lot to the main characters, though he was usually impersonating their beloved ones: he appears to Jack as his late father, to Eko as his late brother, to Ben as his late daughter, to Richard as his late wife... you get the idea.
Roleplay
Apos creates a doppelganger of Bit during one of his battle with the Group.
Toys
In BIONICLE, Makuta first appears to the Toa as a small Matoran. They know it must be some sort of trick, but they are still hesitant to attack him... until he goes One-Winged Angel.
Worse, they do this right after the Big Bad apparently absorbs them both, leaving you to wonder just how fake it is.
Used a few times in a section of the first boss battle in Metal Gear Solid 4, with Laughing Octopus, who uses her OctoCamo and FaceCamo to disguise herself as Naomi, then as Metal Gear Mk. II, with Otacon's voice. When you approach it, it attacks you, with Otacon's voice (although obviously not him speaking) making fun of you.
You can briefly pull this off in Metal Gear Solid 3 thanks to Latex Perfection. Put on the Raikov mask and uniform when fighting his gay lover to distract him for a moment. "Ivan is that you?"
In Star Fox Assault, the final boss tries to pull this, although only with voices from the various people that died (or are presumed dead) during the storyline.
You can attempt one in Majora's Mask against Igos Du Ikana with his knight captain's mask. Then fail completely.
"Ohhh! Keeta! Is it not Captain Keeta?!?... But you're so... T-Tiny!!!"
Also done with the Gorman brothers of the same game while wearing their brother's mask, they get so upset looking at it they can't fight.
Baldur's Gate has at least two examples of this trope: In the Candlekeep catacombs in the first game, and with the Master Wraith in the Throne of Bhaal expansion to the second game (in which the wraith will throw in a Guilt Trip on your Love Interest as well, absolutely free). Amusingly, the character has the ability to outright reject the first instance ("You're no more Gorion than I'm Drizzt the Drow."), but the second instance (about six months, three million XP and a lot of RPG Tropes later) you have to play along for a whole Hannibal Lecture before you're able to throw it off.
If you have a decent Wisdom score, you can point out that you're not buying it in the latter case (but still have to listen through the whole thing).
Not quite an exact use, but referenced: In Paper Mario: The Thousand Year Door, when the Shadow Queen takes over Peach's body Goombella tells you that no matter how much it looks like your friend, you can't think of it that way and just have to fight it.
The Holoprojection droid PROXY in Star WarsThe Force Unleashed uses it to his advantage several times. Going against Vader as Obi-Wan Kenobi or against Boba Fett as his father Jango does not guarantee him victory, but does confuse his opponent for a few precious seconds.
In the Wii/PS2/PSP version of the first game, Darth Phobos briefly adopts the appearance of Juno Eclipse to lure Galen into letting his guard down at the start of the fight. After you defeat her, she tries it again to stop you from killing her. It doesn't work.
In the beginning of the sequel, Darth Vader orders Starkiller to destroy a droid that looks like Juno Eclipse, but he can't. The game's novelization includes a scene where Starkiller is confronted by a Juno droid again, as well as droids of his friends and father, but this time he declares that dreams and memories have no power over him anymore and destroys them all. The Dark Apprentice destroyed his Juno droid and declared that he felt nothing.
Completely averted in Prototype. If Alex is discovered by the military, while shape shifted, the military will not hesitate to fire on him — even if you're in the guise of a respected commander, unarmed woman, or someone they wouldn't dare think about hurting otherwise. Hilariously, if you use the Patsy ability to accuse someone of being Alex, the poor sod gets gunned down with an equal lack of restraint.
In Metroid Gandrayda pulls one of these once you defeat her. First, she changes into Ghor, then into Rundas, then into Samus. The whole thing takes less than a minute, and the only sound that comes out of her mouth is a scream, but the message to Samus is clear: You couldn't save them from The Corruption, you couldn't save me from it, and you're not going to be able to save yourself. Your Days Are Numbered, Samus.
Dante's Inferno: Lucifer takes the shape of Dante, along with his form massacring innocent people in his grand illusions, to break the will of Dante's wife's soul, Beatrice.
In Final Fantasy Tactics Advance, the final totema attempts this in a cut scene. It changes from Ritz, to Doned, to Mewt (all characters with a close relation to the main character), and then finally to the main character himself, each one trying a different tactic to try to get him to stay in the world and not destroy the final crystal. None of it works, though it does succeed in briefly mind fucking the main character.
In Fable III, after having to leave behind your mentor Walter in the deserts of Aurora, the demonic Crawler taunts you with illusions of Walter begging for help and being brutally killed by the Crawler's shadow monster minions.
Not exactly the same, but during a Story Arc of the animated series, a Brainwashed and Crazy Morph shapeshifts into Wolverine's most hated foes (including Sabretooth and Omega Red), trying to take advantage of Wolvie's fractured and tormented memories of said foes to drive him to insanity. Unfortunately for him, Wolvie's got plenty of experience with repressing those memories, and he only escapes by taking advantage of the one memory Wolvie can't repress: being forced to leave Morph to die after a semi-botched mission.
Similarly, Mystique shape shifts into Ms. Marvel to dredge up a bad memory in Rogue of her past to try and guilt her into abandoning the X-Men and hook back up with her; the memory was of Mystique ordering Rogue to fully absorb Marvel's powers and personality, an action that caused Marvel to pop up on occasion as accusatory hallucinations. Rogue eventually had to ditch Mystique as a result, joining the X-Men and having Professor X block her mind of Marvel and the incident to keep the hallucinations at bay, but since he was at this time MIA and unable to keep the block in place, Mystique was trying to convince Rogue that she was the only one in the world who could help her with it.
When the Superman copy is about to be destroyed by Superman, it turns into Lex and says "I think this is the part where you kill me," causing Superman to hesitate long enough for it to (almost) turn the tables.
Notably, The Flash and Batman aren't affected: the Flash drone's attempt to Hannibal Lecture him fails miserably, and Batman defeats his double before it could open its mouth.
Flash's is particularly amusing as he pulls off what is either an obfuscating stupidity Shut Up, Hannibal! or just him being too dumb to understand what his double is saying ("I'm gonna paint my logo on it!")
Green Lantern and Hawkgirl are at first hounded by their doubles, but, having gone through a nasty break-up, decide to switch partners and take the opportunity to "let out some steam".
Wonder Woman's copy doesn't even speak, but she and Superman take a cue from the ex-couple and trade sparring partners, ending with them smashing their doubles into each other.
During Endgame of Transformers Animated, Shockwave tries this on Bulkhead and Bumblebee, asking if they're really willing to take out their old friend Longarm by changing to his form. (In this case, he's only sort of playing head games, as Longarm and Shockwave were always the same person. The Autobots feign sympathy for a minute before smacking him up.
Adventures Of The Galaxy Rangers: One of Mogul the Sorcerer's illusions is that of Zachary's wife. Mogul learns the hard way that this is Zach's fastest-acting Berserk Button. The Queen also pulls this trick on Zach during the Dream Sequence in "Psychocrypt."
Gargoyles: In "The New Olympians," Proteus irritates the prison warden Taurus by taking the form of his father (whom Proteus murdered) and begging to be released from jail.
