A character whose personality isn't marked by any set mood, but by their tendency to swing between moods drastically. Sometimes (if the characters around them are lucky) there's some sort of warning of an impending mood swing — or at least a recognizable cause — but there may just as easily be no warning whatsoever.
Frequently this is combined with emotional lability, so they skip any emotion that isn't extreme. They're never just happy, they're the cheeriest Genki Girl in the world. They're never just sad; they're on the verge of suicide. They're never just angry; they're filled with Unstoppable Rage. And they can flip between any of them at a moment's notice. May very well be a one-person Four-Temperament Ensemble.
Though they may be referred to as "bipolar", their mood swings are generally much more abrupt and frequent than the periods of mania or depression associated with type I of real-life bipolar disorder. This can, however, be a facet of type II bipolar disorder or borderline personality disorder.
This is a stereotypical trait of women who are currently menstruating or suffering from PMS. It is Truth in Television, and fairly common at that, but not typically as extreme as fiction portrays it to be nor as universal. Also, not every woman whose mood is affected by menstrual hormones will be susceptible to every mood; some get stuck swinging into one mood and temporarily become a Fragile Flower or acquire a Hair-Trigger Temper. Since the same hormones are involved, pregnant women are also known for being Mood Swingers.
The Mood Swinger as a Love Interest frequently fills the role of the Tsundere. This is also a common trait of The Caligula.
For even more extreme cases, see Hair-Trigger Temper, Split Personality, Jekyll & Hyde, Tsundere and Yandere. Not to be confused with Mood Whiplash, which is a trait of the storytelling, not an individual character.
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Anime and Manga
Alois Trancy of Black Butler. Throughout the first episode he switches from cute and innocent to Ax-Crazy so many times it could make someone dizzy.
In the beginning at least, Kyo came across as this since he had No Social Skills and had a tendency to snap at people he wasn't mad at. So he'd yell at someone, apologize, then yell at them for not understanding him. Then apologize...
King Hamdo from Now and Then, Here and There takes this trope to terrifying extents. His moods usually switch from petty, to cruel, to violently moody, to pathetic, and back, usually over the course of a few minutes.
A good deal of the characters from Sayonara, Zetsubou-Sensei. Namely Kafuka who's somewhat similar to Rena, except she's even more overly optimistic.
Kamille in Zeta Gundam can go from personable and friendly to teenage angst-fest in the blink of an eye (Not Hyperbole). It's oftentimes difficult to tell what exactly set him off.
Naomi in Zettai Karen Children started off being the calm, sweet girl all the time. Once she realized that her supervisor Tanizaki'streatment of her was why she was having trouble with her powers, she lets loose and from then on shows no restraint in throwing him into a wall when he gets too close. Her Code Name even changes from "Kitty Cat" to "Wild Cat" in the process.
ACDC from Jojos Bizarre Adventure, even admitting that he's the most emotional of the Pillar Men. When Joseph cuts off his arm and destroys it so he can't reattach it, his reaction is to...burst into tears and cry like a little girl, and then after a while stop as quickly as he started and claim he was just blowing off steam. This actually makes him dangerous; Joseph reads his opponents to get the upper hand on them, but it's impossible to read ACDC's constant mood shifts.
Upotte: Agu (a personification of the Steyr AUG) switches from a shy assault rifle to an overbearing squad automatic weapon with a change of barrel/hairstyle.
Sollux is this in canon, but even more so in the fanadventure Be the Sea Dweller Lowblood. He and Karkat will get in a fight, one of them will apologize, the other won't accept, then the second one apologizes and the first one ignores him, they get into another fight, one of them will apologize but be rejected again, one will think about apologizing but change his mind, one will beat himself up over not apologizing sooner but then not go through with it, etc, etc, etc.
Congratulations. Over the span of two minutes, you succeed in getting pissed at Karkat, forgiving him, and getting pissed at him again, despite the fact that you have neither spoken to him nor physically seen him for hours.
This is not bipolarity. Bipolarity is a pendulum swinging back and forth, going tick-tock like the beat of a heart, sometimes slowing so that it will be days before the next tick, perigees before the next tock. Sometimes for a time, it freezes in the middle, and you can breathe easy and clear your head. The emotions are not nearly as neat and tidy as a pendulum, but the process, the process is.
This is not bipolarity.
This is insanity. Not the tick-tock of a pendulum but the tacktacktack of a pinball machine. The swings are jagged and irregular, not back and forth but lurching all over creation. You're lucky when you get to hold on to an emotion for more than a fleeting moment.
You haven't been "bipolar" in sweeps. You miss being bipolar. It was better than this unidentifiable monstrosity.
No it wasn't! Ha ha ha, what are you talking about? It was even MORE horrible compared to this! At least this way you're guaranteed to not have to put up with a bad mood very long.
... Except that you usually switch from one bad mood to another bad mood.
In Mega Man Reawakened, Tron Bonne has been like this since Arc 5 began—and it's revealed it's because she's pregnant.
Film - Animated
Boingo from Hoodwinked acts like this once he drops his Tastes Like Diabetes facade and starts acting like a supervillain. The commentary remarks that on one occasion they couldn't decide which take to use (maniacal, verge of tears, etc), so they just decided to string them all together to very good effect.
After escaping from the tower in which she's lived her whole life without leaving in Tangled, Rapunzel swerves wildly between unrestrained joy at being liberated and crushing guilt at breaking the promise she made to Mother Gothel to never leave the tower.
[ecstatic] I can't believe I did this! [horrified] I can't believe I did this. [overjoyed] I can't believe I did this! [appalled] Mother would be so furious. [sitting by pond] But that's okay, I mean, what she doesn't know who't kill her, right? [sitting in cave] Oh my gosh. This would kill her! [running through leaves] This is sooo funnnn!! [leaning on tree] I am a horrible daughter. I'm going back! [rolling down hill] I am never going back! [facedown in meadow] I am a despicable human being. [swinging from tree] WOO-HOOOO!! Best! Day! Ever! [sitting by rock] *sobbing*
Basil from The Great Mouse Detective certainly swings from manically happy to crushingly depressed and back to manically happy again quickly. Of course, this is because he's based on Sherlock Holmes, who was possibly bipolar, and definitely a massive cocaine addict. So either Basil could use a little lithium, or he's gotten into a bad, mouse-sized vial of seven per cent solution (an expected hazard of living under Holmes' floor).
Multiple characters in The Room, but Johnny most obviously. He (in)famously goes from a tirade about how his "future wife" has falsely accused him of domestic abuse to being happy to see his best friend.
Tonker in Monstrous Regiment has only two moods: calm and berserk. The description used in the book is "has no middle gears."
Peter Pan: This is Tinker Bell's original characterization. J.M. Barrie explains that fairies are 'so small they only have room for one feeling at a time.'
"Now, Tinker Bell was not all bad. At least, she was not all bad all the time."
Hell, Peter's like this at least as much. Having been raised by fairies may have something to do with it.
Less than halfway through the first novel, it's easy to see where theories of Sherlock Holmes being bipolar come from.
Edward Cullen of Twilight fame is a rather offputting example. He's portrayed as a tortured, yet romantic gentleman who has the utmost devotion to Bella...but only when she's submissive. However, when she puts up even the slightest resistance or actually voices complaints, he quickly goes from a "sensitive" boyfriend to a frothing, rageful lunatic bent on putting her in her place. In addition to his manipulative tendencies, it is because of this that he serves as the perfect candidate for a walking PSA and redflag archive on Domestic Abuse.
Live Action TV
Dick from 3rd Rock from the Sun: "I find you pompous, judgemental, and completely self-absorbed. Would you be my friend?"
Not really an example; that was more Dick relating to those qualities because he's the exact same way.
The wife of the man with the bazooka round in his chest in the bomb squad episode of Gray's Anatomy. She goes from uncontrollable screaming to uncontrollable tears to uncontrollable rage and then back to uncontrollable tears.
Muppets Tonight director Nigel suffers from this, going from mellow to hysterical and back again at the drop of a hat. Though working with The Muppets can have that affect on people.
Sherlock's Jim Moriarty has, to say the least, a habit of doing this. "SORRY BOYS! I'M SOOOOOO CHANGEABLE!"
Tyres from Spaced does this frequently, with the camera angle often changing with his mood. Explained as a long-term side-effect of taking waaaaaaaaay too much ecstasy.
The Caesars depicts Caligula as one of these; Caligula even says that he uses this to his advantage so that people do not know whether a given act of flattery will please him or anger him, thereby keeping them afraid of him.
Cat in Victorious can go from happy to sad in under 4 sentences.
A pregnancy example from The Nanny. The combination of her pregnancy and her son moving away to college left Fran's emotions all over the place, pinballing around in seconds.
Niles: I thought tonight for a celebratory dinner, I would make salmon.
Fran:[Morose] Oh, no, those poor little salmons. They spend their whole life swimming upstream just to be poached for dinner?
Niles: I was going to barbecue.
Fran:[Chipper] Ooh, mesquite? With a teriyaki glaze? That would be perfect for Brighton's celebratory dinner. [Crying] my baby's leaving. [Angry] Who put their finger in the cake!?
Relient K: Whoever the song "Mood Rings" was about: "First she's Jekyll and then she's Hyde... at least she makes a lovely pair", indeed.
Malfeas in Exalted is known to be bipolar on a huge scale, but the real Mood-Swinger among the Yozis is Kimbery, whose affections change like the tides - as soon as she's disappointed by you at all, she goes loveloveloveloveloveSOMUCHHATE. Being in the second category is a death sentence; the first, a death sentence suspended until you inevitably enter category two.
One of possible monsters in Munchkin Card Game is a "bipolar bear" - he will attack you furiously or run away, depending on its current mood.
Lucy from 13 has this dialogue with her boyfriend:
Lucy: ...And don't forget to change your facebook status to in a relationship. That way our profiles will be linked together like little love handcuffs. Brett: (To Cassie, who is walking by) Oh hi Cassie. Lucy: Are you flirting with her?! Brett: No, I was just saying Hi. Lucy: So you're saying I'm fat? Brett: No. Lucy: Well, you never say anything about how amazing I look anymore. Brett: Lucy, you are driving me crazy. Lucy: See? Now that was sweet. And sweet makes me sweet. Got it? Brett: No.
The Dance Dance Revolution announcer, or at least the one from DDR Hottest Party. He lavishes praise on you when you're doing well or even just okay, but the second your dance meter falls into the red he'll angrily demand that you stop sucking. Also, when you idle on the song menu he demonstrates all the patience of a spoiled six-year-old ("Bo-ring!").
Anders of Dragon Age II is stated in the Codex to suffer from manic and depressive phases as of Act III, due to the Sanity Slippage caused by his Demonic Possession. His dialogue throughout the game tends to reflect this, ranging from calm and caring, to snarky, to obsessively focused on his goals, to self-righteously grandiose, to self-loathing and miserable, to downright psycho, with alarming speed.
Due to programming, Cole Phelps from L.A. Noire. During interrogations, he'll typically ask a calm, polite question, and then if you select "Doubt" or "Lie", abruptly start screaming obscenities at the suspect, then return to normal for the next question.
Persona 3: While not as extreme as some of the others on the list, Yukari Takeba frequently goes through random moodswings through the course of the game. Said mood swings get worse in The Answer thanks to her grieving over the main character's death.
Perfectly understandable too, given the fact that, before the events of the game, she was (for the most part) a normal teenager.
In Psychonauts, you delve into the brain of a former actress to control her mood swings. Her mind is represented by a stage where can change the atmosphere from cheery to depressing with a switch.
Soul Series: Tira is either bloodthirsty-cheerful or bloodthirsty-angry. She can change semi-randomly during a fight.
In her second appearance this escalated from Bipolar, to actual split personalities and her fighting style changes depending the current mood/personality.
Arguably all characters in the Galactic Adventures expansion for SPORE, as players are only given 5 emotions to work with (happy, sad, angry, scared, and neutral which is just a different version of happy) when making dialogue portions for their user-created missions, and they're all expressed in the most exaggerated way possible without and transitions between them. This also makes certain character types such as the Deadpan Snarker and The Stoic very difficult to portray believably.
Zimmy is similar, though her personality is a bit more subdued. (Read: not psychotic.) Her mood swings revolve around her friend Gamma. An in-depth analysis of Zimmy's issues on the matter would make this page about twice as long, but the short version is this: Gamma acts as a grounding wire for the bulk of Zimmy's Reality Warper powers, which she can't control, and thus keeps Zimmy's mind mostly at peace. Zimmy is nonetheless aware that she's not a lot of fun to be around, and is terrified that if Gamma thought she had any other friends, she'd spend her time with them instead of Zimmy; between this fear, and the power Gamma can't drain away, her overall reaction to the world is erratic and designed to keep people at arms' length by whatever means necessary.
Sollux's ancestor-descendant Mituna also turns the intensity of Sollux's mood swings up a few notches, swinging erratically between being a raving, foulmouthedJerk Ass and a cowed, apologetic Extreme Doormat. It may not have always been so bad, seeing as he suffered a Heroic RROD that severely crippled his mind before his introduction.
The 20 Questions-playing Akinator can switch between smug satisfaction and red-hot rage between one question and the next, if what he thought was a perfect guess is thrown off by an answer that doesn't fit the bill.
Critic: A BAT CREDIT CARD???????!!! I'LL KILL ALL OF YOU!
*3 hours later*
Critic: Sorry. I just get a little crazy when I see a BAT CREDIT CAR-!!!
*12 hours later*
Critic: ...RAPE MY CHILDHOOD YOU-
Played for Drama in Demo Reel with Tom Collins. During a phone call message to Donnie, he goes from jovially commenting that SWAG is pretty awesome, to full-blown-stalker threats telling Donnie to not even think about ignoring him.
Terry: I just want you all to know that I don't care if you're a boy penguin or a girl penguin, I just... Aw, heck, I'm just gonna come right out and say it: I love you all to pieces! Penguin: Oh, thanks Terry. We all love yo— Terry:GO TO HELL!! *Runs off crying*
Avatar: The Last Airbender : Theories/jokes about Katara being bipolar really took off after a scene in the ninth episode (after some hints in the first episode) where she explodes at Aang, becomes apologetic in the space of a few seconds ("Oh, my gosh, Aang, I'm so sorry! I don't know what came over me!"), only to turn around a few seconds later and angrily insist "No more apologies!" After her behavior in "The Awakening," a pervasive meme became "must be her time of the month."
Also Prince Zuko. One minute, he's calm, focused, and disciplined. The next, exploding in a flurry of grunts, growls, and fireballs. Even as a good guy, his mood doesn't improve much until the end. This is why he cannot use Lightning Bending, which requires absolute control over one's emotions.
Lunch Lady from Danny Phantom who switches from kind, grandmotherly figure to full-blown "I'll KEEL you" mode at the drop of a hat. Her alternate future daughter picked up the habit as well.
(sees dead bunny) "You... sick... bastard." (squishes dead bunny, merrily sends the bunny's kidnapper off in a rocket ship, and holds dead bunny up) "Heeey, little guy! Underneath all that precious fur..." (cuddles dead bunny mournfully) "All scared and all alone..." (pets dead bunny contentedly, starts singing) "If only~ they knew~ what it was like~ to be~ yyyyoooOOOOUUUU!" (rips dead bunny's skin off and gleefully wears it as a hat) "Jared, I want one of these for each and every inmate, not a moment to spare!"
Lola Bunny from The Looney Tunes Show has a tendency to have little control over her emotions, one moments she can be crying her eyes out and the next she's laughing her ass off. Being a Cloud Cuckoolander, it could be attributed to her never being entirely sure what's going on around her.
Made fun of in one episode of The Simpsons. Homer takes sleeping pills to sleep well through the night and Lisa reads off the side-effects, one which is Mood Swings. Cue Homer repeatedly saying "Mood Swings" in various moods.
In the original seriesGalvatron would switch moods at a drop of a hat. His moods seemed either insanely gleeful, terrified, sarcastic, calm or enraged. His moodiness is a symptom of his damaged metaprocessor (along with irrational behaviour).
Peg from Goof Troop can switch from being sweet and happy to terrifyingly angry in the same sentence, and effortlessly move back and forth between them. This usually occurs because she's addressing two parties at once, one she has a beef with and one she doesn't.
Rapid cycling bipolar disorder (in which the characteristic mood swings take place in periods of days or even hours, rather than weeks as is more typical) somewhat resembles this trope, and as noted above most fictional characters with bipolar disorder are depicted this way despite it being comparatively rare.
One of the key symptoms of Borderline Personality Disorder are rapid mood swings that can happen within hours. note As stated in the DSM-IV manual : "6. Affective instability due to a marked reactivity of mood (e.g., intense episodic dysphoria, irritability or anxiety usually lasting a few hours and only rarely more than a few days)". This trope is so much a core indicator of that disorder, in fact, that some propose to change its name to "Emotional (dys)regulation disorder" because the term "Borderline" is outdated and carries too much social stigma.
To be clear, the key difference between BPD and bipolar disorder is that mood swings in BPD are always a response to external stimuli (even if it is something as trivial as your pen being out of ink) while mood swings in bipolar disorder are spontaneous.
Some people with PTSD have mood swings as part of the hyperarousal aspect of the disorder.
Ivan the Terrible had mood swings as a side effect of the quicksilver he used to treat his pain, one bipolar episode had him beat his son to death in a fit of rage. He reacted with devestating sadness.