"I embrace my rival, but only to strangle him."Two characters are (or were) clearly friends; the innocent lead's friend probably had a dark past or was shunned by others, but nonetheless they managed to become friends. However at some point, their playful rivalry starts to escalate (a one-sided affair, usually) or they simply drift apart because of the plot. Often times the powder keg is a single event that breaks the increasingly tenuous friendship, at which point one friend becomes the villain. This can be part of a newly revealed dark past, or dealing with a major problem in a way his friend or other people can't tolerate. Sometimes it's just a very minor misunderstanding no one seems interested in clarifying. This is a common fate for The Rival and certain types of Lancers, especially if they were the lead character's friend and Missed the Call, or if they grew up with a third, female friend. If the main character is a Messianic Archetype or even just has a strong connection with his True Companions, expect him to make it his mission to turn his friend back to the side of good even while everyone else is trying to kill him. Whether he succeeds or fails in this mission becomes a source of much drama for the character. Meanwhile the rival turned villain's job is to dredge up all his pent-up feelings of jealousy to try to justify his actions often becoming his own worst enemy in the process. If the rival is a sibling, it's Cain and Abel. Contrast Rivals Team Up and Driven by Envy. Sister trope of The Rival. May or may not be The Resenter. Compare Arch-Enemy. See also Evil Former Friend.
— Jean Racine
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- Afro and Jinno in Afro Samurai. Interestingly, it's the protagonist, Afro, who "turned evil" — Everything Jinno does is to protect those at Swordmaster's dojo, while Afro is motivated purely by revenge.
- Nina and Arika's humorous Odd Couple relationship in Mai-Otome disintegrated due to a number of...misunderstandings, to put it mildly. They started out as rivals in academics as a result of Garderobe only allowing a limited number of students to advance, but over time, the rivalry is over the love of Nina's adoptive father Sergay. When it turns out that Sergay is anonymously supporting Arika, Nina becomes Nagi's Otome and tries to kill Arika.
- Likewise, Miaka and Yui in Fushigi Yuugi.
- Yui and Haruna in Corrector Yui. Though Haruna gets better more or less quickly.
- Sasuke and Naruto in Naruto is the definition of this. Interestingly enough, though, the rivalry started out one-sided and became mutual as Naruto got stronger.
- Spike and Vicious in Cowboy Bebop, although this is a probably an inversion - Vicious didn't turn evil (it's pretty clear he was always a sociopath) so much as Spike turned good. Or at least turned neutral.
- Actually, it is implied that Vicious didn't use to be this much of a sociopath, and Spike's betrayal pushed him over the edge.
- Tabool to Nabuca in Now and Then, Here and There. Winds up killing his best friend. Ample moments of Foe Yay make the turn even more disturbing.
- Code Geass:
- Lelouch is an inverted form of this trope. Lelouch's philosophical disagreement with Suzaku over whether to destroy Britannia or fix it from within leads to the two childhood friends at gunpoint in the Season 1 finale. In R2, Suzaku and Lelouch come to the conclusion that there's a better option than destroying Britannia or fixing it from within doing both.
- Temporarily reversed in R2 when Suzaku is seen to be playing for Charles but instead of them turning on each other, Suzaku eventually joins forces with Lelouch to put an end to the wars altogether.
- Megumi from Gate Keepers who turned traitor because she envied Ruriko's seeming superiority.
- Kaiser Ryo in Yu-Gi-Oh! GX occupied an odd spot in between The Ace and The Rival in season 1, before having a Freak Out in season 2. Oddly enough, despite at least two attempts to break him out of it, neither succeeded. In fact, he gets better all on his own. Even before that, he's still something of a jerkass protagonist in the second half of season three.
- And speaking of Yu-Gi-Oh, Yu-Gi-Oh! 5D's's Kiryu Kyosuke was the lead's best friend as the leader of Team Satisfaction, but he got too...excited by the whole conquering-Satellite thing. When he tried to duel all of Security (the police force) and ended up killing one of them, leading to a situation where he believed that Yusei had turned him into the police, Yusei's and Kiryu's friendship got shot to hell. Kiryu died, which ironically meant that he got better and everything else got worse at the same time. And by everything else, that means he got so Ax-Crazy you could make a case of him reaching critical, Dilandau Albatou levels.
- Also from 5D's, Jack Atlas. Prior to the series setting he was also a member of Team Satisfaction, and a friendly rival to Yusei. He grew more and more distant from his friends following the break-up of the team, and eventually accepted an offer to become King in return for betraying Yusei by stealing his D-Wheel and Stardust Dragon, which he accomplished by endangering Rally's life. He's by far a less extreme example than Kiryu, though, as this was the last truly evil action he took, and once Yusei settled their score he quickly became a protagonist and sought redemption in the second season.
- Kenshiro and Shin from Fist of the North Star were friends from different martial art schools who were in love with the same woman until Jagi, Kenshiro's jealous brother, swayed Shin into betraying Kenshiro and taking Yuria for himself.
- Played with in Glass Mask. When both actresses whom are competing for the iconic role of Crimson Goddess arrive on the island where the film has always taken place, Ayumi notices a weak wooden bridge ready to give out and is smart enough not to cross it. Later on, Maya, her rival for the role, is not as Genre Savvy as Ayumi and tries to cross the bridge. Ayumi sees this and thinks that letting Maya die would finally eliminate the threat to her future super stardom. But she changes her mind at the last minute, right when the bridge does give out, and saves Maya's life. She confesses to Maya about it out of guilt, which lead to the two girls fighting and making it clear where they stand with each other out of respect.
- In the anime version of Dragonaut: The Resonance, Kazuki and Jin started out as friends. Jin was forced to drop out of the program they were both working in during an accident which ultimately resulted in Kazuki's Dragon rejecting him and accepting Jin. When Kazuki finally gets a Dragon and ends up fighting Jin, he gets badly scarred, causing him to hate Jin even more.
- In the Ice and Snow arc of D.N.Angel, Kyle, Elliot's best friend does this because of the Two Guys and a Girl scenario. He's eventually doomed a la Rime Of The Ancient Mariner to tell the story so others don't make his mistake after he causes BOTH of their deaths. Sort of. It's complicated.
- Kenichi: The Mightiest Disciple works this in, but it's among the many things Kenichi has *forgotten* since his childhood. Big Bad Odin is really a childhood friend who made a promise with Kenichi that they would both get stronger...before insisting on fighting over a pin Kenichi received. Odin reveals that it was the loss in that childhood fight, coupled with the humiliation of being offered the pin afterward anyway, that set him on the Path of Asura.
- In Mobile Suit Gundam Wing, Milliardo and Treize, but it is more of Civil Warcraft
- In Macross Frontier, Ranka temporariliy turns evil, at least from the human perspective, due to Mind Control. Then she breaks out of it with the help of Sheryl and in return saves her life
- Enchu for Muhyo in Muhyo and Roji. Initially, his grades were far superior to Muhyo's because he put in effort while Muhyo did not, but as Muhyo's talent awoke, he began surpassing Enchu. This was a particularly sore spot for Enchu, as he hoped to become an Executor to support his sick mother. Losing the position to Muhyo shortly after losing his mother broke Enchu, and resulted in his Face–Heel Turn. It turns out that Teeki invoked this trope to find a powerful body to take as his own, and killed Enchu's mother to turn him evil.
- Cardfight!! Vanguard loves this trope. In Kai's backstory, he becomes friends with Ren Suzugamori after transferring to the same school which Ren attends. Ren soon obtains Psyqualia and begins losing his sanity before the start of the series, while Kai, having no idea how to save his friend, decides to leave Ren behind and move back to the town where Aichi lives. Kai walking out on Ren actually made things worse. Then, we move on to the beginning of the story proper, where Kai has taken up a cold and distant approach in an attempt to keep history from repeating. He does end up joining a team with Aichi Sendou, only for Aichi to start developing Psyqualia and losing his sanity as well. Thankfully, they both regain their sanity by the end of the first season.
- However, in the third season, Kai is the one to turn evil this time, as he winds up being reversed after his fight with a reversed Takuto and going so far as to take up the very clan that embodies Void.
- In the Buu Saga of Dragon Ball Z, Vegeta had enough of being Always Second Best to Goku, so he willingly let Babidi brainwash him so he can get the power he needed to fight Goku once and for all. Goku only had one day on earth due to being dead and Vegeta would never have another chance at besting him and threatened to kill anyone until Goku gave in. This unfortunately set off a chain of events that led to Buu's resurrection due to the intensity of their duel.
- Doctor Doom and Reed Richards are probably one of the most straightforward examples of this trope. The catalyst for Doom turning evil was an incident where Reed pointed out a flaw in one of Doom's projects when they were both classmates in grad school rivaling for the spot of top student, and Doom, thinking Reed was just jealous, ignored him, only for the project to blow up in his face, disfiguring him and getting him expelled. Doom was so infuriated by the idea that that infernal Richards had bested him that he rejected that version of reality, blamed Reed for the accident, and dedicated himself to destroying Reed's life in "revenge".note
The "rivalry" part of their relationship (in addition to the blinding, obsessive vendetta) is still in full force, as the reasons for all of Doom's evil plans to Take Over the World, gain cosmic power, topple governments, decimate the superhero community, and rain death and destruction upon all oppose him can eventually be traced back to his desire to get one over on his old college pal. (He'll even delay killing Reed to make him admit Doom is better than he is.)
- In the Spider-Man comics, Eddie Brock was a reporter that developed a grudge against Spiderman when the latter apprehended Sin-Eater. Brock had been interviewing a compulsive confessor and lost his job in disgrace. In addition to his job loss and inability to find a new one in journalism barring tabloids, he was also diagnosed with terminal cancer, so when he encountered the symbiote (which had tracked Brock by his negative emotions and the cancer upon which it feeds) in the church where Spiderman had abandoned it, he was more than willing to use the symbiote to get the revenge he craved... armed with the knowledge of Spiderman's secret identity and calling himself Venom in reference to the types of articles he was paid to write. He ultimately reformed and gave up the symbiote knowing that without it he would eventually die of the cancer ravaging his body. Except... the trace of the symbiote in his blood fused with white blood cells and created the menacing Anti-Venom, who, while not as evil, and fights villains like the current Venom Mac Garnagan (AKA: The Scorpion), still holds a grudge with ole Spidey. Oh well.
- This was inverted in the 70's Marvel Universe comic Shang-Chi, Master of Kung Fu. As an acolyte of the despotic ambitions of his father, Fu Manchu, Shang-Chi was always just a little better than his childhood friend, Midnight. But in this case it was Shang-Chi who turned, rejecting his father's evil and leaving the organization. Naturally, his old friend got sent after him. Said friend added a righteous sense of betrayal to his previous resentment.
- Legion of Super-Heroes: Some versions of Lightning Lad's relationship with his brother Mekt (Lightning Lord) go this way.
- Subverted in ElfQuest. Rayek starts out as Cutter's rival of sorts. Several times in the story, he appears to be about to turn evil, but despite his screwups, he never does, at least, not permanently. Rival Turned Heel–Face Revolving Door?
- The Superman villain Conduit was once a childhood rival and friend of Clark Kent but tried to kill him years later.
- So was Lex Luthor, at least in the Silver Age continuity. Though he had a friendly rivalry going with Superboy not Clark.
- Iron Man:
- Tony Stark gets one for a short time in the form of Tiberius Stone, who's implied to have been a sociopath from the beginning, just very good at hiding it. "Short time" because by the end of the arc, Stone has been left comatose, and tends to be forgotten by later writers.
- Obidiah Stane also served this purpose.
- Doctor Strange had Baron Mordo, until he reformed while dying of cancer. And then he was brought back as a villain because the writers either forgot about his reformation or were just too lazy to create a new evil sorcerer.
- In Wolverine's limited series Wolverine: Origins, in the 19th century, Dog starts out as James Howlett's friend and secret half-brother, but eventually they drift apart, and Dog even kills James' puppy. James eventually runs away from home after an incident causes the death of his parents and Dog's father, and Dog tells the locals it was James' fault. Years later, Dog is told to track James down so that James' grandfather can make amends, but instead Dog tries to kill James for taking his name (Logan). This one also works for Two Guys and a Girl, with James, Dog, and Rose. Dog is later tracked down by Sabretooth, Logan's other rival turned evil when he ends up in the present, and is hired as a teacher at the Hellfire Academy.
- Archie Comics' Sonic the Hedgehog: Sonic's old rival Geoffrey St. John was revealed to be working for Ixis Naugus, and is implied to have been working for him for some time. As it turned out, Geoffrey has always worked for Naugus — as in, before either was even introduced into the comic — so this isn't so much "Rival Turned Evil" as it is "Rival Who's Always Been Evil But Playing Good".
- Catwoman: Cassandra Cartland grew up in the same orphanage as Selina Kyle where Selina rebuffed Cassandra's attempts at friendship. Cassandra grew bitter and resentful and, after learning that Selina had become Catwoman, created her own villainous identity of She-cat and attempted to frame Selina for crimes she committed.
- In Shaman's Tears, Thom Broadarrow was a childhood rival to Joshua Brand. He was always jealous of Joshua, especially when Joshua got the girl they both wanted, then abandoned her to leave the reservation; something Thom never managed. When Joshua, returned to the reservation and was granted the power of Wakan Tanka, Thom thought it should have rightfully be his. His jealousy ultimately led to accident that transformed into a monster.
- Alex to Marceline.
- Maylu's Revenge has Maylu herself becoming this to Roll.
- Nami and Elly in Pirate Meets Gebler when the latter used the Drive to not only fight Fei, but also the former. They got better after the former got the latter to snap out of this by threatening to drop herself off the canyon..
- Eddie Brock is portrayed this way in Spider-Man 3, as fighting for Peter's position at Jameson's office. The Rival/Evil Turn comes after he tries to cheat his way ahead and Peter exposes him; Peter, wearing the Venom suit, goes to a local chapel seeking repentance, while Eddie goes to the same seeking revenge; the church bells ring, disturbing Venom sufficiently that Peter is able to fight him off, and Venom drops down a few stories to take over Eddie, who is a much more willing host...
- While he and Ip Man never became friends, Jin descended from martial artist rivalry to becoming a bandit and later selling Ip Man's last known whereabouts to the Japanese. He does a Heel–Face Turn back by the second film.
- Ben and Ian in the first National Treasure movie.
- The 2002 film version of The Count of Monte Cristo with Guy Pierce and Jim Caviezel differs from the book by making Dantes and Mondego good friends with a friendly rivalry before Mondego betrays Dantes out of jealousy.
- The escalation of this effect drives the plot of The Prestige. Robert Angier and Alfred Borden are both amateur magicians working fairly amicably as audience plants for another magician. However, when Angier's wife is killed and Borden appears to be responsible (it's possible that he's entirely innocent) Angier devotes himself to destroying Borden by any means including sabotaging and stealing his tricks as well as killing himself dozens of times over just so he can eventually frame Borden for murder and take his daughter. Borden remains far from innocent though and retaliates by constantly ruining Angier's chance at fame and stealing his mistress.
- Like his comic counterpart, Iron Man had his fair share of evil rivals. The first was Obidiah Stane who, in the Marvel Cinematic Universe was an in-company rival to Stark Enterprises as opposed to the head of his own company. He eventually turned evil and tried to kill Stark. In the second film, Justin Hammer was a business rival who ended up releasing an international criminal who had previously killed many people and tried to kill Stark, in order to defeat Stark Enterprises.
- Iceman and Pyro in X-Men.
- Overlaps with Love Makes You Evil in The Sorcerer's Apprentice. Maxim Horvath was a friend of Balthazar Blake and was in love with Veronica Gorloisen, with all three being Merlin's apprentices. Veronica chose Balthazar, resulting in Horvath deciding that he no longer cared about anything and would willingly help Morgana destroy the world and is not shy about trying to kill Balthazar. Centuries later, Balthazar is surprised that Horvath still feels bitter about Veronica's choice.
- Harry Potter:
- James Potter and Severus Snape never got along at school, made worse by the fact that Lily, Snape's childhood friend who he was thoroughly in love with, ended up marrying James, while Snape turned evil. Snape turned back to the side of good because Lily was in danger, and stayed on the good side because his previous evil actions caused her death.
- Played With: Harry's own rival, Draco Malfoy, was pretty petty in the early books, but when Voldemort returns to power, he joins him. He winds up causing a lot of damage in the sixth book, but he also can't bring himself to actually murder Dumbledore, and he's clearly terrified of all of the actual psychopaths whom he's forced to work with. He never technically switches back to the good side until Voldemort's defeat, but he's so scared and pathetic that Harry briefly pities him.
- The Inheritance Cycle: Murtagh pulls a Face–Heel Turn while everyone else thought he was dead. Supposedly this is because the Big Bad knows his true name, but he also displays some jealousy of his brother Eragon. Unlike other examples, he is surprisingly reasonable about it, including sparing Eragon's life.
- Saruman in The Lord of the Rings turned to the Dark Side because he was jealous of Gandalf having received the Ring of Fire from Círdan upon their arrival in Middle-Earth.
- Happens with Brutus in Con Iggulden's Emperor series. He starts out as Caesar's loyal blood brother, but as time goes by he grows increasingly jealous of his friend's success as a general and a politician. Eventually he leaves Caesar to fight for Pompey in the civil war, and tells Caesar later that he did it because he wanted to be someone without having to stand in the shadow of the great Caesar.
- Faquarl of The Bartimaeus Trilogy was already evil, but he's The Rival of the titular character and a White Hair, Black Heart, so he fits.
- At the end of Graham McNeill's Warhammer 40,000 Ultramarines novel Dead Sky Black Sun, Honsou corners the Space Marine Vaanes, who had helped Uriel against Honsou before deciding that it cost too much. Honsou offers to let him work for the Chaos forces and promises Revenge, and Vaanes accepts. Sequel Hook, anyone?
- It seems as if half the Forsaken in The Wheel of Time had been very talented and effective good guy generals and Aes Sedai who could not stand that their rival Lews Therin Telamon (whom they were at least fairly friendly/neutral colleagues with at one stage), The Dragon, was better than them, thus turning to the Dark One (Not the case for other Forsaken like the sadistic, and unparalleled healer, Semirhage, or The Philosopher Ishmael).
- Played most straight with the Forsaken Demandred. His entire life can essentially be described as "almost as good as Lews Therin". He was born a day later. He was ever so slightly weaker in the Power, and ever so slightly less proficient at its use. He was slightly shorter. He was slightly worse looking. Ilenya chose Lews Therin over him. He was passed up for becoming the Amyrlin Seat in favour of Lews Therin. Lews Therin's deeds in the War of Power were slightly more impressive. So on and so forth. Characters from the time who think back say that in any other time, in any other place, he could have been the greatest hero of the age, but he had the misfortune to be born into Lews Therin's time. He fell to the Dark primarily out of jealously for Lews Therin, and to this day is described as the Forsaken with the most hate(particularly for Lews Therin, but in general as well). Not the most anger, not the biggest ego, not the most sadistic. He just hates.
- Pasha in Doctor Zhivago, though he was Lara's friend (and thus more of a romantic rival for Yuri).
- Septimus Heap: Simon Heap became an antagonist after Septimus got the ExtraOrdinary Apprenticeship.
- Breezepelt from Warrior Cats was originally the rival to Lionblaze and Jayfeather, but he was friendly enough to team up with them on occasion. However, in Omen Of The Stars he is recruited by the Dark Forest and turns homicidal.
- In "Song of the Lioness," Alex becomes this to Alanna
Live Action TV
- Buffy the Vampire Slayer: Buffy and Faith interacted this way, sliding in and out of rivalry as the story progressed.
- The Master and The Rani from Doctor Who are two examples as well.
- In Kamen Rider 555, the two main characters, Inui Takumi and Kiba Yuuji, switch back and forth between friends and enemies, not once but several times. Eventually, Kiba does a Face–Heel Turn and becomes an all-out villain, but in the last episode he returns to the Light Side and does a Redemption Equals Death.
- The early seasons of Smallville dealt with Clark and Lex's relationship this way.
- 24's Jack Bauer seems to have an endless string of these, particularly Christopher Henderson.
- The Vampire Diaries' Damon got his Start of Darkness this way. The antagonistic relationship between him and Stefan began with their rivalry for the affections of Katherine.
- Justified: U.S. Marshal Raylan Givens and Harlan County marijuana dealer Dickie Bennett were high school baseball rivals, whose enmity turned deeply ugly when Dickie attacked Raylan during a game, starting a fight that left Dickie with a permanent limp and an undying grudge, and nearly reignited a long-buried feud between the Bennett clan and the Givens' family. Decades later Raylan and Dickie find themselves on a collision course again, and for Dickie at least, It's Personal.
- Rizzoli & Isles: Alice Sands, the Big Bad in season 6 (and part of season 7) attended the police academy with Jane. Alice was second in her class behind Jane, after having been the best at everything prior to then. This created her dangerous fixation on Jane. She dropped out of the academy and became a drug dealer. Years later she would return, determined to destroy Jane's life before killing her.
- "Enemy" by Fozzy apparently is about two people who were once friends and then had a breakup of some kind and the lyrics talk about they will always be on opposite sides.
- Low Ki and Xavier had feuded and made up for years beforehand, but Xavier had officially crossed over to evil when he joined The Prophecy to abolish the honor code and remake ROH in the image of Christopher Daniels. Also part of the Prophecy was Samoa Joe, a token good teammate Only in It for the Money who refused to violate the honor code, which made Low Ki the rival turned evil when he went on a hiatus from the company and returned as a member of the Rottweilers, spitting on Joe's ROH Championship belt after he defeated Xavier.
- Sora and Riku in Kingdom Hearts (pictured above). Riku turns to the dark side by More Than Mind Control, and is eventually possessed by the Big Bad. After he's freed from this possession at the end of the game, he spends Chain of Memories and Kingdom Hearts II dealing with the consequences. It's especially interesting that Riku fits all three of the marker tropes at the top of the page: he was supposed to have the Keyblade, has very nearly white hair, and grew up with Sora and Kairi.
- Street Fighter: Sagat turned to crime and hatred after losing a fight to Ryu (and getting hideously scarred in the process - In his defense, Ryu did attack him with a cheap shot, while under the influence of the Satsui No Hado). He turned good again at the end of Street Fighter Zero/Alpha 3.
- Caim and Inuart in Drakengard. Inuart is helped along his Face–Heel Turn by More Than Mind Control.
- One of the plotlines of Breath of Fire: Dragon Quarter mostly follows this trope. Ryu and Bosch start the game, while not necessarily as mutual friends, at least as co-workers. When Bosch starts observing (and once ends up on the wrong end of) Ryu's suddenly-manifested dragon powers, he jealously begins resorting to increasingly twisted means to match that power, including getting a freaky cybernetic body and, eventually, gaining similar powers from an evil dragon, after it sensed his selfish motives.
- In the backstory for City of Heroes / City of Villains, the men who would become archnemeses Statesman and Lord Recluse were once very good friends. When they drank from the Well of the Furies, one became an Incarnate of Zeus, got lightning powers, and named himself Statesman, while the other became an Incarnate of Tartarus, grew spider arms, and named himself Lord Recluse. One of them became an evil overlord.
- Ryudo and Melfice in Grandia II were in fact brothers, but like everybody from their soon-to-be-cursed village very competitive. Melfice turns out to be possessed by the Horns of Valmar, forcing the brothers to fight. Additionally, Ryudo meets up with his childhood friend and rival who has to admit he's become soft and Ryudo is currently a better swordsman than him, at least until he starts training again.
- Cecil and Kain in Final Fantasy IV. Kain was being (at least partially) mind-controlled, and he repents eventually, but his jealousy over Cecil (mostly regarding Rosa) makes him that much easier to control.
- Valis: Yuko and Reiko.
- EarthBound: Ness and Pokey, though they weren't ever really friends. (Ness is on better terms with his brother.) Understatement. The guy starts out cowardly, bitchy, and self-centered (you see the type in media being self-aggrandizing when the danger has passed). Things go downhill with him from there. Oddly, he doesn't seem to know what he wants to be as far as Ness goes. In MOTHER 3, you can find Ness' yo-yo in Porky's playroom in the thunder tower, it's named "Friend's Yoyo".
- Lyon of Fire Emblem: The Sacred Stones.
- Maxim in Castlevania: Harmony of Dissonance, but only because of manifestation of Dracula took his body over.
- Before that, Hugh Baldwin in Castlevania: Circle of the Moon. He's the son of his and Nathan's master, and is jealous enough of Nathan getting the Hunter whip that he turns evil and is fought as the second-to-last boss.
- Straybow in the Medieval chapter of Live A Live; jealous of his childhood friend Oersted's fame and success, he ultimately orchestrates a plan intended to thoroughly disgrace and break him. Whether he intended it or not, he ultimately ends up causing Oersted to undergo a major Face–Heel Turn, declaring that Humans Are Bastards and becoming the game's Big Bad. So, in the end, both rivals turn evil.
- Songi starts as Gala's rival in Legend of Legaia. He quickly turns more and more depraved and villainous in his journey for greater power.
- Dist from Tales of the Abyss fits this trope perfectly.
- Dr. Wily from Mega Man fits this trope.
- Revya becomes this to Danette during the Demon Path in Soul Nomad & the World Eaters.
- Steiner is blatantly set up for this in Luminous Arc 2, although it's worth noting that from his viewpoint, the hero has turned evil (having collaborated with several criminals, one of whom was an accomplice to a mass murderer.) The two go through the standard "redeem each other or kill each other" schtick before Steiner sacrifices himself to kill a greater villain.
- Jake from Ape Escape is turned into Spike's enemy by Spectre' brainwashing.
- Dirk in Suikoden Tierkreis is a mentor turned evil—not an Evil Mentor, but a formerly good mentor who succumbs to a combination of fear, suspicion, and jealousy after his proteges a): suddenly acquire magical powers they never worked to earn, and b): start to remember things he's certain never existed. He starts out just trying to snap them out of it, but winds up barely coherent.
- Krauser is this one in Resident Evil 4. The prequel, Resident Evil: The Darkside Chronicles puts up this explanation.
- Shadow from the Sonic the Hedgehog series becomes this to Sonic in his own game if you choose to join Black Doom by completing the one of the dark missions in G.U.N. Fortress, Black Comet, or Final Haunt.
- Tsubaki Yayoi towards Noel Vermilion in BlazBlue. Thanks to a certain Manipulative Bastard, Tsubaki grew to resent Noel for having the position as the secretary of her Love Interest. To twist it even further, she finds out that in another timeline, she would have had said position if Noel never existed. Thankfully, they resolve their issues by the time of the third game and reconcile.
- Strider 2 introduces Hien to the series, a fellow Strider and close comrade of Hiryu who shared several missions together. Hien's jealousy of his friend's seemingly unreachable ability, however, pushed him to side with the Grandmaster and cause the dead of all Striders save Hiryu. Hien still considers Hiryu a friend and even tries to talk him out of fighting the Grandmaster, citing how futile his one-man crusade is, but Hiryu is determined to finish his mission and simply tells him to get serious next time they meet.
- Seifer Almasy in Final Fantasy VIII, who starts off as a Jerkass rival to protagonist Squall Leonhart before going all in and siding with the Sorceress Edea.
- Yashu from Half-Minute Hero: The Second Coming. His rivalry with the main hero Yusha starts out friendly, but Lamde (an antagonist from the previous game) exploits his fanatical devotion to Queen Maria and, through a series of machinations and tragic misunderstandings, causes Yashu to go berserk and swear revenge on Yusha and all of his descendants.
- Peter Parker and Eddie Brock in The Spectacular Spider-Man were childhood friends, but over the course of the series they drift further apart, as Eddie's grudge becomes greater, leading to their destined battle as Spider-Man and Venom.
- In Transformers Prime Optimus and Megatron were once friends (hardly rivals) as Orion Pax and Megatronus while they worked together to progress civil rights on Cybertron. It didn't end well. Optimus still thinks he can bring his old friend back until towards the end of the first season.
- Splinter (Hamato Yoshi) and the Shredder (Oroku Saki) as seen in the 2012 Teenage Mutant Ninja Turtles. They were once good friends until they both fell in love for the same women, Tang Shen. Their rivalry for her love soon turned into hatred, which ultimately killed Shen and Splinter's infant daughter.
- In Trixie's second appearance on My Little Pony: Friendship Is Magic, she becomes this to Twilight under the influence of an Artifact of Doom.
- Danny Phantom:
- Jack Fenton and Vlad Masters, although the former is unaware of this case until the Grand Finale.
- Tucker becomes this in "What You Want", going too far with his ghost powers and attempting to outshine Danny about it. He eventually gets better from it.
- The Adventures of Jimmy Neutron: Boy Genius has an unusual case in the episode, "The N-Men". For this example, the titular character is the rival turned evil being angered on how Cindy keeps on overshadowing him. This eventually sets him off and he goes on a rampage. Ironically, the only person who can calm him down was Cindy, who he was originally mad at.
- Morph becomes this in X-Men season 2. Wanting revenge on the X-Men for leaving him to die, his acts in the beginning of season 2 includes posing as a reverend to lead Scott and Jean away for Mr. Sinister to get them, locking Beast away in the Danger Room and increase the danger, and getting Storm shot while posing as a police officer. Then, it's a subverted when he has a Split Personality doing all this. He eventually gets better from this by the season 2 finale.
- Phil from Ben 10 in the beginning of season 2.
- Cy-Kill is this to Leader-1 in Challenge Of The Go Bots.