Soko from Ao no Fuuin, though not to the extreme as most other examples. Due to her status, she keeps feeling torn between the humans and the Oni. She often, almost instinctively, saves her fellow Oni from the humans when they are attacked, but is overall opposed to creating more Oni and returns to the humans' side to continue to reach her goal.
A character in the anime version of the Mushi King arcade, called Soma. He is at first suspicious, but becomes good, then becomes a baddie to be with his mother, who's one of them, and then becomes good again when everyone but The Dragon sees how ludicrous the Big Bad's plan is. Really, this guy gets the Revolving Door award.
Vegeta from Dragon Ball Z. First he was a villain, then he joined Goku and the other Z Fighters in the fight against Freeza... then he turned back into a villain in a Not Brainwashed moment during the Majin Buu saga... and then became a good guy once again.
In Dragon Ball GT, he was brainwashed and fused with the new Big Bad, Baby. Then he was good again. The series ended before he could rotate any more.
InuYasha: Kikyo can't seem to decide if she wants Naraku dead, or Inuyasha, or both. She also has difficulty deciding why she wants it done and how to do it.
Zabine Chareaux from Gundam F91 starts out as a villain, but by the point of Crossbone Gundam he's a good guy. Then he switches sides again halfway through that one.
Also in Gundam Wing, Char's "greatest fan" Milliardo Peacecraft switches in almost the exact same pattern that Char did, only he does it all in one season.
EVERY character in Wing likes to take a spin in the revolving door. The Gundam pilots usually opt to either act idependently or work for the "good" faction, whatever it might be (Sanc Kingdom, Peacemillion, Preventers), though Trowa joins both Oz and Mariameia to spy on them, while Wufei joins Marimaia for awhile. Zechs however goes from Earth Forces to OZ, to independent, to trying to join the Gundam Pilots but not making it to the battle in time, before finally joining White Fang, and then back to helping the Gundam Pilots for real. Trieze doesn't so much betray Oz as them betraying him, but then later deciding they need him after all and begging him to come back.
Naruto: Not actual good-to-evil-to-good-etc flip, but the character Sasuke has done enough 90 degree side turns that he's more or less constantly one turn away from being Naruto's ally again, although he generally chooses to go further down the spiral towards the Moral Event Horizon instead.
Viletta Nu from Code Geass. She started as an enemy soldier, then got amnesia and became Ohgi's love interest, then recovered her memory and non-fatally shot Ohgi, then was recruited in the second season to spy on Lelouch while posing as a teacher, then got blackmailed by Lelouch into working for him while pretending to still be secretly spying on him. When Jeremiah showed up hunting for Lelouch, she revealed his location eagerly and begged Jeremiah to free her from Lelouch's control, but as soon as he left, she contacted Lelouch to warn him that Jeremiah was coming for him, giving him time to set up a trap. She later arranged a meeting with Ohgi, intending to kill him, but ended up reconciling with him and eventually betrayed Lelouch alongside him. After Lelouch dies, they both got married. Can she do ballet spins on those High Heels?
To a lesser extent, Suzaku. Though exactly what side he starts on is debatable. Starts with Britannia, despite their anti-Eleven policy, though he begs them to spare his old friend, Lelouch. Then joins Euphemia, who while Britannian, has a completely different idea to the rest of Britannia, then when Euphemia dies, he joins Emperor Charles specifically as a Britannian super-elite, while his home country of Japan suffers even worse than before, then tries to join Schneizel and become Knight Of One (the most super elite) in return for killing his former boss, Charles, then joins Lelouch, who actually had killed Charles, in order to kill Lelouch himself. Sure, for most of that, he's on the opposite side to Lelouch, and he's always on the opposite side of Kallen, but exactly who's side he's on changes pretty often.
Making it even more bizarre, the guy killed his own father seven years before the events of the series itself, but did it because he thought it was the right thing to do, then joined Brittania!
Gajeel from Fairy Tail seems to be turning into this, but he's been leaning to the Face side recently... He starts out as a mercenary for a rival guild, but gets defeated and then is convinced to join Fairy Tail by Juvia, a guild member from said rival guild who did a permanent Heel–Face Turn. After supposedly proving that he was on their side by doing a small show and protecting Levy (whom he had gravely injured in the previously mentioned attack on the guild) from an attack, he shows himself (although not to the guild) to be The Mole for another contractor who also aims to take down Fairy Tail. Once Laxus' attempt to forcibly rebuild the guild to his liking is foiled, he goes to his new employer and finds out his plan. He returns to the guild, where he tells Makarov of his employer's location and his plan, apparently working as a double agent.
The Team Rocket trio from Pokémon loves this trope. Some episodes just have them doing casual work with no intention to bother anybody, but they go back to more dishonest work later.
They tend to do a complete 360 in the films: turning good to help Ash and co., and then returning to crime once the crisis passes.
Beato. Gets rather Moe at points, has a very weirdTsundere-ish relationship with Battler, and has a lot of Pet the Dog moments, some going so far as to resemble a Heroic Sacrifice when she's faced with Eva-Beatrice. She appears to be aware of this trope, and will joyfully trip back and forth across it just for the lulz.
Crona from Soul Eater looks like he might be falling into this trope (at least in the manga). When we first meet Crona, he was the first reoccurring antagonist. After the Kishin revival arc, Crona does a Heel–Face Turn and helps Shibusen—or at least intends to. Then Medusa uses this as an opportunity to turn Crona into The Mole, which he is very reluctant to go along with. The anime leaves Crona with the good guys. The manga has Crona captured by Medusa. As of the latest chapters, Crona has been once again driven to madness and is against Shibusen—or really, anyone who's not with Medusa... which is just about everyone (except Free and a reluctant Eruka and Mizune).
And to make it worse, as of Chapter 87, Crona has turned on Medusa, killing her because of her (apparent) kindness towards him (saying that his mother would never be so kind to him).
Gets even worse when near the end s/he manages to absorb Ashura, and becomes the Big Bad, however this is reverted in a few chapters later when his/her will starts to falter, allowing Ashura to take control again and Chrona ultimately helps the good guys in the finale, by sealing herself/himself and Ashura inside the moon for good...well maybe, depending if you believe Maka when she said she'd come back for Crona.
Ryo Mashiba from Hajime no Ippo is introduced as a straight up heel. He gets somewhat better after his loss against Ippo, even getting his own fandom. Then, he fights Ryuuhei Sawamura, who's not only even worse than him but also tried to hurt Mashibas sister. In the fight, he completely loses the veneer of humanity he had, gloats over said fact and that he silenced his own fandom with his terrible actions, and goes against the rules so bad that he's disqualified. The next time we see him in a fight, he refuses to cheat despite being angrier than a thousand devils (since his opponent is cheating) and even grudgingly throws his boxing gloves to the crowd.
Hintata from Bloody Cross seems to keep changing sides, one minute it seems like he's a good guy who genuinely wants to help Tsukimiya, even getting past her lie detecting powers, the next minute he's betraying her and trying to kill her. Tsukimiya once commented that it was as if there were two Hinatas, and she was never sure with version of him she would encounter. It turns out his behavior is because he's a Fake Defector and had to fight her to maintain his cover.
Horrorman from Anpanman. Sometimes he'll hang with Baikinman and the other villains, other times (especially when he gets a starring episode), he'll do his own thing and be shown in a positive light. However, he's always cemented on the villains' side in the movies. Even his Image Song states that you shouldn't put too much trust in him, as he could easily betray you!
If you're a female character in any of the Pretty Cure series and you're becoming close to one or more of the heroines, be prepared to spend some time on the Revolving Door. Just ask Kaoru, Michiru, Setsuna, Ellen and now Regina.
Regina is more of a deconstruction than anything; every time she goes bad, it's clear that it happensagainsther will. King Jikochuureally hates defection. In fact, the only reason why she went bad again in episode 22 is because unlike Setsuna, she has a very weak heart more prone to manipulation, whereas Setsuna has the strength to not only pull off a White Frag but also resist Northa's hypnosis long enough for Pretty Cure to save the day.
Yui from Fushigi Yuugi tends to have this. Starting as good, she turned bad when she joined forces with Nakago, but she turned good again near the end of the series, combined with Redemption Equals Life, although at the cost of her life at first. However, it doesn't end like this; in OVA 1 episode 2, she is Brainwashed and Crazy into becoming the priestess of Genbu. In the next episode, she's good once again after breaking free from the trance.
Homura is an anti-heroic version of this trope, before playing it straight in Rebellion. Originally, she was a Naďve Newcomer and mostly stayed on the sidelines, until Madoka's first death pushed her to become an Action Girl. Her numerous failed attempts to save Madoka made her incredibly cynical, and by the beginning of the series, she's a far cry from the kindhearted Shrinking Violet she once was, constantly antagonizing the other girls. Once her Hidden Depths are revealed and Madoka relieves her of her trial, she becomes idealistic once more, until Kyubey pushes her off the slippery slope and she becomes a demon.
Kyoko is a more straightforward example of this trope. Starting out as an idealistic Magical Girl, her father going mad from the revelationbroke her and turned her into a selfish loner who picks fights with other and has a Social Darwinist philosophy. Her interactions with Sayaka help her grow into a more heroic, if not cynical, attitude, and she ends the series on good terms with her former mentor, Mami.
Sora from .hack//SIGN starts off as a wandering player killer, but from there begins to help out whoever he thinks will offer the most interesting results. By the end of the series he's joined up with every faction at least once. Justified in that's he's a fourth grader playing for fun, and since it's just a game there's no real consequences to anything he does. At least until he angers one powerful person too many...
Ymir from Attack on Titan is.....capricious at best, always playing her cards extremely close to her chest. Utterly devoted to Krista, she'll pretty much do whatever she thinks is best to keep her safe. This results in her rapidly switching sides over the course of a few chapters, going back and forth based on which side she believes can best keep Krista safe at the moment. In the end, she settles for returning Krista to the Survey Corps before going to rescue Reiner and Bertolt. It's probably best to just say she's on Krista's side, and leave it at that.