[[quoteright:330:[[Anime/SkyGirls http://static.tvtropes.org/pmwiki/pub/images/red_girl_blue_girl_2450.jpg]]]]

->'''Detective Somerset:''' We must divorce ourselves from emotion here. No matter how hard it is, we have to remain focused on the details--\\
'''Detective Mills:''' Hey, man, I feed off my emotions, ok? How's that? [Mills promptly gets into a fight with a reporter snooping around the crime scene] Hey, man, I'm sorry. They just... they piss me off, ok?\\
'''Detective Somerset:''' It's ok. It's impressive to see a man feeding off his emotions.
-->-- ''Film/{{Se7en}}''

This trope is about two individuals (or occasionally, factions or civilizations) that are significantly linked together somehow (such as through a rivalry, friendship, being [[SiblingYinYang siblings]], co-workers, competitors, etc.) that have differing personalities and, with it, often different approaches to the world in general, especially whatever they might be competing in/for/over. This, of course, gives them something to butt heads about, since their manner of acting may rub each other the wrong way and each is sure that their way of doing things is right.

By the way, don't get too stuck on the colors in the title. The title refers to a Japanese tradition that gets elaborated a bit later in the article, but it doesn't mean that a character has to dress in or be associated with a particular color in order to fit the role.

Their personalities usually run roughly along the following lines:

The '''Red Oni''' is associated with [[HotBlooded passion, wildness, and defiance]]. A red oni character is often more brawny than brainy, extroverted, enthusiastic, determined, and filled with a zest for life. He is also much more likely to break conventions and rules than his counterpart. Often an IdiotHero or, in more mature pieces, a BoisterousBruiser. If a red oni is indeed noticeably intelligent, he will still be much more emotional than the blue oni and/or with less sophisticated thoughts, feelings, or goals. When a villain, the red oni is often a BloodKnight. Generally speaking, they're more people-oriented than goal-oriented (although both Oni's can be either). They may or may not shrug off criticism about themselves or their aims and goals, but say or do something nasty to their [[TrueCompanions closest friends]] and you've just guaranteed yourself [[RoaringRampageOfRevenge a world of pain]]. Likely candidate for TheMcCoy. In terms of character alignment, the red oni tends towards the Chaotic alignments, though this isn't always the case.

The '''Blue Oni''' is associated with serenity, control and observance of authority. A blue oni is more intellectual, proud, traditional, introverted, and cultured (sometimes more spiritual, although that's not guaranteed). Blue oni personalities are often respected by others, but also likely to puzzle or confound their peers because they are difficult to read and have a mysterious quality to them. Personality subtypes include TheStoic, TheSpock, TheQuietOne, and the AloofBigBrother. It's worth noting that while blue types ''appear'' tremendously calm and composed, sometimes, this is just the surface, and scratching that façade may show that they are, if anything, wound '''far''' too tight beneath their calm and unemotional appearance. Villainous blue oni tend to be extremely ruthless, ready to do or sacrifice anything (and often anyone) to accomplish their goals. See KnightTemplar and WellIntentionedExtremist. In terms of character alignment, the blue oni tends towards the Lawful alignments though this isn't always the case.

While many expressions of this trope play it straight, it is also not uncommon for the Blue Oni and Red Oni to mix traits. The Red Oni, while usually the more passionate and emotion-driven, may also be the more easy-going and flexible of the two. His/her more simplistic world-view may make them at least partially [[AngstWhatAngst immune to angst]] and {{Wangst}}. Conversely, the Blue Oni, while usually stoic and thoughtful, may hid a volatile, angsty nature that may erupt if [[HeroicBSOD pushed]] [[BerserkButton too far]]. In such instances the two may switch roles, with the Red Oni becoming the reassuring, composed partner and the Blue Oni becoming a [[UnstoppableRage Whirling Dervish of Rage]]! (Compare pairings such as Red Oni Ken (cocky, arrogant, but decidedly easy-going) and Blue Oni Ryu (stoic, disciplined, but brimming with internal conflict).

If ElementalPowers are present, you can usually expect the red oni to use PlayingWithFire or something of a similarly flashy nature, while the blue oni will prefer MakingASplash or possibly be AnIcePerson and an ElementalRivalry is born.

There are a lot of ways to play their interactions, so sometimes the two are both leads on an equal standing, sometimes the blue is more of TheSmartGuy or TheLancer to the red's {{Hot Blood}}ed TheLeader or TheBigGuy. The relationship can also vary wildly, as they are just as likely to be close friends as they are to being bitter rivals or enemies. One thing that is sure is that regardless of how big a cast is, when these two are in it, they will be magnetically drawn to each other.

Note that many times, this will be a relative, not an absolute, distinction: the Red Oni need not be TheFool and the Blue Oni need not be a StrawVulcan.

The name comes from traditional Japanese folklore where {{Oni}} (think troll/ogre) usually came in two flavors, a red and a blue. Over time, it's gotten popular to ascribe different qualities and personalities to the two colors. (Think the high-strung Type A personality and the easy-going Type B personality.) See also the Marid and Ifrits over at OurGeniesAreDifferent.

In a show where it's relevant, differences in fighting style may be something like TechnicianVersusPerformer. For example, a red may rush headlong into combat without focus or a plan, relying on suprise or intimidation. It may be that their strategy is to close the fight quickly before their opponent can think or adapt. It's also possible that they're going to tank their opponent's defenses in order to focus on beating them down. The blue may use [[StealthExpert cunning traps, stealth and strategies,]] or tend to win in elegant SingleStrokeBattle fashion, or rely on hitting a target with magical powers or projectiles from a distance instead of running up to their opponent and punching them in the face. ForceAndFinesse is a variation where red tends to rely on raw power while blue uses precision strikes. [[Analysis/RedOniBlueOni Analysis page]] has more examples of possible red-blue dynamics.

Add one person that bridges the Red Oni and Blue Oni and you got FreudianTrio, with the Red Oni becomes the Id , the Blue Oni becomes Superego, and the connecting person becomes the Ego. That's the basic formula, but not always. Sometimes as the plot progresses, someone new is added and as they become the trio, this newcomer might be even more emotional than the Red Oni or is even more calm and calculating than the Blue Oni. And whoever keeps the trio together is the ego.

Compare OutsideManInsideMan, OddCouple, CastCalculus (for the evolution of the Red/Blue dynamic through various numbers of ensembles), TomboyAndGirlyGirl, SensitiveGuyAndManlyMan, and OrderVersusChaos. See also PinkGirlBlueBoy and ActionDuo. If the contrast in personalities between the two actually brings them to blows, you probably have a TigerVersusDragon situation. If they are friends it will often be of the VitriolicBestBuds variety, most commonly where one character is completely oblivious to the vitriol.
!!!All examples are listed in the red-then-blue order unless otherwise specified:
* RedOniBlueOni/{{Advertising}}
* RedOniBlueOni/AnimeAndManga
* RedOniBlueOni/CardGames
* RedOniBlueOni/ComicBooks
* RedOniBlueOni/{{Fanfic}}
* RedOniBlueOni/{{Film}}
* RedOniBlueOni/{{Literature}}
* RedOniBlueOni/LiveActionTV
* RedOniBlueOni/{{Meta}}
* RedOniBlueOni/{{Music}}
* RedOniBlueOni/MythologyAndFolklore
* RedOniBlueOni/NewspaperComics
* RedOniBlueOni/ProfessionalWrestling
* RedOniBlueOni/{{Religion}}
* RedOniBlueOni/{{Sports}}
* RedOniBlueOni/TabletopGames
* RedOniBlueOni/{{Theatre}}
* RedOniBlueOni/{{Toys}}
* RedOniBlueOni/VideoGames
* RedOniBlueOni/VisualNovels
* RedOniBlueOni/{{Webcomics}}
* RedOniBlueOni/WebOriginal
* RedOniBlueOni/WesternAnimation
* RedOniBlueOni/RealLife