He used to be a villain, and he's now working for the heroes. However, the only reason he's in the good side or at least being friendly to the heroes is because he forgot his own identity and his past, and that he's supposed to be the enemy of the heroes.
If the villain remembers his past he will suffer from an identity crisis, questioning his real identity, which may lead to several outcomes:
- The villain suffers from a mental breakdown.
- The villain immediately becomes evil again and turns against the heroes.
- The villain realizes the damage he had done and decides to pull a fully-conscious HeelFace Turn to redeem himself.
In some cases, especially if the amnesia happens toward the very end, the villain would never be able to remember his past, and this is often being considered to be a makeshift HeelFace Turn due to amnesiac people are often treated as harmless, innocent people. Often a cause of Relationship Reboot, as long as the villain in question stays amnesiac.
There are several factors that can affect the outcome. If the villain in question is treated with kindness by the heroes, he may take The Power of Friendship into consideration, especially if he never felt love and friendship before. If he's treated badly by the heroes, either because they're not convinced that he's now on their side or by one person who repeatedly rejects his attempts to do good, he may decide that it's better off to stay evil, sometimes becoming exactly what they accused him of. In some cases, the villain will stay evil out of loyalty to his own organization, refusing to betray the other villains who trusted him. Last but not least, whether the villain would realize his past deed was damaging.
Sometimes, the heroes will hope that the said villain won't regain his past memories. They will actively try to stop him from remembering anything, feeding him Fake Memories, or even brainwashing him into becoming good. Taking it one step further, they will even try to eliminate the villain before he can remember anything. If the heroes have different opinions on how to dispose of the said villain, it may lead to a conflict or even a flat-out infight between them.
Compare Amnesiacs are Innocent, where amnesiac people, in general, are treated as innocent. Contrast Amnesiac Hero, where the hero suffers from amnesia, and Criminal Amnesiac, where a hero suffering from amnesia is being convinced that they're supposed to be a villain.
- Light Yagami of Death Note does this as part of his Memory Gambit, having given up his memories of being Kira and joining the Kira taskforce, even working together with his nemesis L. Light without the Death Note is actually a pretty decent guy, but we know that this cannot last, and sure enough, after a very convoluted Gambit Roulette, Light reclaims the Death Note and regains his memories and everything goes to hell.
- Curaçao from Detective Conannote was a Black Organization operative and right-hand agent of Rum who was responsible for the deaths of three spies in the Organization. She lost her memories after being sniped off the highway while being pursued by the authorities and was found by Conan and the Detective Boys. Their decency towards her as she helps Conan and the Detective Boys with the case that she started convinces her to defect from the Organization for good, even after regaining her memories. She dies saving them and other civilians from a collapsing Ferris Wheel.
- Zig-Zagged with Jellal in Fairy Tail, who was merely Brainwashed and Crazy before getting amnesia, and assists the heroes to atone for the things he did as a villain. But when he regains his memories of Natsu, the one who defeated him, it appears that Jellal has slipped back into his evil ways until he delivers him a Power-Up, proving that he's truly on the heroes' side, and stays that way once his memories fully return.
- Happens temporarily in One Piece to Charlotte "Big Mom" Linlin (or "O-Lin" as she goes by on Wano) after King sinks her ship. Without her memories, Big Mom reverts to the genuinely kind personality she had as a young child, though she keeps her monstrous strength (leading to a few hilarious moments). It doesn't last long, as her memories return when Queen sucker punches her (much to the poor guy's horror).
- Happy Heroes has at least two examples.
- The first couple of episodes of Season 2 (which form a Multi-Part Episode) inverts this trope by having Careful S., one of the heroes, lose his memory and end up working for the villains Big M. and Little M.
- Speaking of Big M., episode 50 of that same season plays the trope straight by having him forget he's a villain and help the heroes defeat a shark that he himself sent out earlier.
- In Pleasant Goat and Big Big Wolf: Joys of Seasons episode 95, an amnesiac Wolffy, fully convinced he's Weslie the goat, not only acts friendly towards the actual goats but tries to defend them from the other wolves. It doesn't end well for him, as the other wolves win the fight and drag him off.
- Uncanny X-Men had an example in their Utopia era when Rich Bitch villain Sebastian Shaw lost his memories and was taken in by the team led by Hope Summers. He played surprisingly well with Hope and the kids, but eventually the amnesia wore off and he returned to the life of a Corrupt Corporate Executive.
- The Phoenix has this as its premise. Zuko, having been scarred and banished by his father the Fire Lord, ends up nearly drowning with a head-injury saving one of his crewmen during a storm. He washes ashore with amnesia and is found by Team Avatar, the very group he was looking for for all of these years. He joins them, unaware of his past or mission, and by the time he remembers who he is, he has decided that helping the Avatar defeat his father would be the best course action for everybody.
- Super Robot Wars GQ: Sara Zabiarov's only clue to her true identity is her Bolinoak Sammahnnote and she has joined the Turbines before they met Tekkadan, but she didn't become one of Naze's women.
- At the end of Kubo and the Two Strings, after the Moon King/Raiden is brought down into human form, he loses all memory of who he is or any of the things he's done, despite having tried to kill Kubo only minutes earlier. Despite this, Kubo and the townspeople decide to respect Raiden and tell him that he was a great person. He is very happy to hear this and decides to devote himself to "continuing" his generous lifestyle.
- Steven Universe: The Movie: Big Bad Spinel is hit with her own rejuvenator early on in the movie, reverting her to her original form: a harmless (if eccentric) playmate for Pink Diamond, now Steven. She sticks around with Steven for most of the movie, but unfortunately Steven needs to restore her memory to deactivate the injector she planted on Earth.
- A variation occurs in The Outer Limits (1995) episode "Blank Slate". At the beginning of the episode, an amnesiac man asks a woman for help, because he's being pursued by some MIBs'. Throughout the episode, as they're on the run he uploads some missing memories into his brain and discovers that at some point in the past he had been carrying out some questionable experiments. As he's about to upload his final memories, the woman tells him that maybe he shouldn't upload them, that maybe he erased those memories because he didn't want to remember them anymore, and he destroys them. Just, then the MIBs' catch up to them, and tell the man that his experiment on erasing memories was a success, and to go back to the lab and continue his research. When they refuse, the MIB's overpower the couple, and the episode closes with the man having uploaded his memories and the woman strapped to a chair, begging him not to experiment on her, only for him to say she should be thanking him for making her forget something she doesn't want to remember anymore.
- Agarest Senki: Vashtor is the second-in-command of the armies of darkness. However, after his defeat at the end of the First Generation, he is thrust off a cliff and gets washed ashore on a different continent with none of his memories. After that, the heroes stumble upon him and nurse him back to health, not knowing his true identity. He then joins the heroes in their fight against the darkness, becoming a party member in the process. He does betray the heroes once he regains his memories, but you still have a chance to redeem him later.
- Dragonball Z Budokai Tenkaichi features a What If? saga titled "Fateful Brothers" in which Goku's evil brother Raditz suffered amnesia like Goku himself did in Dragon Ball and became a good guy for a while. Being an adult, the Easy Amnesia eventually wears off, and after waffling back and forth between his newfound heroism and original villainy he ultimately leaves Earth and collides his Attack Ball with Vegeta's in a Heroic Sacrifice to protect his brother one last time.
- Heavily implied in AI: The Somnium Files. Kaname Date lost his memories six years before the game began, which is also when the Cyclops Killings took place. His boss and coworker both insist that the events are a coincidence and that he has nothing to do with the case. It's ultimately subverted. He inhabits the body of the Cyclops Killer due to a case of Grand Theft Me that caused him to lose his memories, but he was on the heroes' side the whole time.
- This is part of The Reveal in Knights of the Old Republic: the player character is actually Darth Revan, who had his/her mind wiped by the Jedi Council to ensure that (s)he would no longer be a threat to the galaxy. Following this, the player can decide whether to remain heroic or else take over as the new Dark Lord of the Sith.
- Mortal Kombat: Before the events of Mortal Kombat 3. Kabal was a member of Black Dragon who was victim of Shao Kahn's extermination forces and left for dead. He was disfigured, the reason why he has to use a gas mask permanently, and with his memories gone joining to Earthrealm forces against Shao Kahn. He eventually regained his memories after MK3, as seen in his next appearance in Mortal Kombat: Deception, in which he not just comes back to Black Dragon clan, he also wants to remake the organization from the ashes.
- The twist of the emotional rollercoaster known as MySims SkyHeroes is that the player character used to be the chief enforcer for the evil Morcubus, before getting shot out of the sky and left amnesiac to be unwittingly picked up by the resistance against Morcubus.
- The postgame of Pokémon Mystery Dungeon: Explorers of Time, Darkness, and Sky has you defeat Darkrai, The Man Behind the Man of the main plot. After his defeat, Darkrai loses his memories, and can be found wandering random dungeons where he can then be recruited into the team.
- In Super Robot Wars Advance, Axel Almer was an Ace Pilot of the invading Shadow-Mirror army, but lost his memories in the transition to the heroes' dimension and got picked up by them first.
- Perceval, a Dark-type Blade that can join your party in Xenoblade Chronicles 2, is revealed to have previously been a ruthless assassin before having resonated with his current Driver. However, Blades do not retain any memory of their former 'lives' under different Drivers, and, this time around, he operates under a strict Thou Shalt Not Kill policy (though he is nevertheless devout in his efforts to punish evildoers).
- In the Adventures of Sonic the Hedgehog episode, "Blank-Headed Eagle", Scratch chases after Sonic with an Eagle-Copter kit he ordered from watching the Edgar Eagle TV show. After Sonic tricks him into shooting himself out of the sky, Scratch gets amnesia, and when Henrietta Peck finds him, Scratch thinks he's Edgar Eagle. Sonic takes advantage of this and uses Scratch to help him save Farmer Peck's farm from destruction from Dr. Robotnik's machines. Scratch ends up regaining his memories near the end of the episode.
- Class of the Titans: After Big Bad Cronus and all the team fall into the River Lethe and lose their memories, Cronus becomes totally amicable and joins the gang in trying to work out what's going on.
- Xiaolin Showdown: The Season 2 finale has Jack Spicer accidentally turn himself good when he brought the Reversing Mirror into the Yin Yang World, reversing his own spirit in the process. He remains with the heroes until needing to enter the Yin Yang World again, resetting his spirit back to evil (but not before creating a good clone of himself in the meantime.)
- In the Skylanders Academy episode "Sheep(ball) Dreams", the Skylanders attempt to have an amnesiac Dreamcatcher leave the Doom Raiders and rejoin them.
- In Lolirock Praxina lost her memory during a fight, and Iris used her amnesia to try and reform her. She actually winds up succeeding, but Praxina willingly restores her memory and becomes evil again after seeing how heartbroken her brother Mephisto was over her joining the good guys.