An (almost) literal contrast in 80's animated film The Transformers: The Movie: Hot Rod, impulsive and flame-themed (well, his name is a hint), is red (in a color tone tending a bit to magenta). Kup, calmer and more experient, is greyish blue (or blueish gray).
Marvel Comics continuity in particular played up the contrast between Blue Oni Autobot leader Optimus Prime (calm, noble and rational) and Red Oni Dinobot (and sometimes all Autobots) leader Grimlock (impulsive, violent and bombastic).
Also in Transformers' eyes. The wild, violent Decepticon have red eyes; The good-hearted, rigtheous Autobots have blue eyes.
The two factions in general. The Decepticon's are the Red Oni dominated side (with a few Blue Oni exceptions like Soundwave), while the Autobot's are the Blue Oni opposites (with a few Red Oni exceptions like Jazz).
The Boondocks: Huey Freeman is a blue oni in the Ryu mould (reserved and scholarly whilst holding radical political views with great passion) while his brother Riley is a Ken-style red oni (loud and charismatic but in his own way much more of a people person).
Robin and Starfire are represented by Red and are more brash, stubborn, and passionate than the others (Starfire displays this most in the comics, to which the flashback/origin episode "Go" paid tribute).
Cyborg and Raven are represented by Blue and are more intelligent, analytical, and, in Raven's case, cold.
In the Teen Titans comic, Red Devil (literally a Red Oni) and Blue Beetle have become a comic relief dynamic duo that fans have dubbed Beevil. While both characters have similar personalities, Beetle tends to be level headed while Devil is more impulsive.
Slade is definitely the villainous Blue to Robin's heroic Red.
Probably the most clear cut case for this in actual characters is the contrast of angry, defiant Determinator Zuko, who lives and dies with the Indy Ploy (although he is by far one of most complex reds), and the Chessmaster/Dragon Azula, who even makes blue fire (which is actually hotter). Some other notable examples of playing with red vs blue include:
Zuko's Dream Sequence, where his Uncle Iroh is represented by a red dragon and Azula by a blue one.
Best friends turned enemies Avatar Roku and Firelord Sozin both had dragon companions. Roku's was red, Sozin's blue. Sozin tricks Roku, leaving him to his death.
Zuko also plays Red Oni to every one of Team Avatar's Blue Oni's (except for Toph).
Aang and Zuko are taught a different version of Firebending philosophy, formed, at least in part, by a red and blue dragon. Zuko is being taught to move beyond destructive motivations (his red oni background) while Aang is being taught to accept his destructive potential (against his blue oni background). Also the contrast in their facial markings. Zuko's Red Scar and Aang's Blue Tattoos.
Zuko and Katara in general Zuko is angry and passionate while Katara is caring and nurturing, however in the interactions between just the two of them, they switch roles. Katara is constantly angry and emotional and extremely protective of Aang and her own interests. Zuko is calm and thinks about how to deal with her. Katara is typically dressed in blue and Zuko in red.
Except, of course, in each of their 'spirit' alter-egoes: the Blue Spirit and Katara's reddish Painted Lady. In that commonality, their color schemes reverse.
During the Boiling Rock infiltration Sokka was the Blue Oni, A non bender Water Tribesmen strategist and The Leader vs Zuko's Red, a Firebending Fire Nation fighter and The Lancer.
Aang is, appropriately enough, a combination of the two. Aang normally can be both foolish and passionate (represented by his normal saffron clothes and temporary Fire Nation wardrobe change, which he later salvages to make a new set of robes). But when in his glowing blue Avatar State, Aang can become frighteningly cold and ruthless, to the point where Aang himself has nightmares about it.
More broadly, this applies to the Fire Nation and Water Tribe. Fire Nation are typically dressed in red (and black) compared to the Water Tribe, who are typically dressed in blue (and white or gray). Firebeinding is offensive, Waterbending is mostly defensive.
In terms of personality this is inverted in the case of the vastly giftedmotherless young daughters of important leaders on the opposing sides. Katara a passionate and somewhat impulsive Team Mom whose will sabotage her own plans to help people in need, while Azula is a calculating manipulator who has a hard time dealing with people on a personal level at all.
The Siege of the North - the Fire Nation troops are the aggressive attackers whose deluded military leaders (General Zhao and Prince Zuko) are busy with insane schemes (to kill the Moon spirit and capture the avatar respectively). Meanwhile, the Northern Water Tribe are defending themselves, planning to infiltrate the Fire Nation navy]] and seek help from the spirit world. The color scheme throughout those episodes fits this trope to a t - the moon and night sky become blood red when the Fire Nation Admiral Zhao begins to kill the Moon Spirit, foolishly risking the destruction of the earth in order to prevent water benders from bending, thereby making the city an easy target. The response from the Northern Water Tribe is exceptionally non-aggressive, as the princess, something of an incarnate of the Moon Spirit, sacrifices her mortal life with the aim of restoring balance to the world and enabling others to fight against Fire Nation imperialism.
Also, the (red) Fire Nation and the (green, not blue) Earth Kingdom during the attacks and sieges of Ba Sing Se and the establishment of Fire Nation colonies, where the Fire Nation are pretty clearly the (unsustainably) aggressive party. Of course, the Earth Kingdom has the Dai Li, who dress in cooler colors and engage in more underhanded aggression, like brainwashing.
Its sequel series, The Legend of Korra, features hero Korra and her lancer Mako who also double as the Official Couple (for the first two seasons anyway) as this trope. Their Chromatic Arrangement is the opposite of their oni; Korra, the Avatar, is a hotheaded native waterbender, and Mako is a cool and controlled firebender.
Korra is also the brash, rough-and-rumble Red Oni to Asami's calm and elegant blue. Just like with Korra and Mako, their Chromatic Arrangement is the opposite of their oni, with Asami consistently dressing in red and Korra primarily dressing in blue. They also double as the Official Couple at the series finale.
In season 2, Varrick and his quiet assistant Zhu Li played very straightly. Also Desna and Eska count, Eska becomes a rather terrifying Red (matching or even more scarier than Azula), while Desna plays a submissive and somewhat more understanding Blue twin.
Keith and Lance, as the Red and Blue Paladins respectively, embody this trope in battle. Keith is impulsive, hotblooded, and reckless, while Lance is strategic and level-headed. Ironically, outside of combat, they reverse this trope. Keith is stoic, reserved, and pragmatic, while Lance is passionate, emotive, and extroverted.
Although, at first, Stan and Kyle had similar personalities, later on, they were differenced. Stan was an introvert, calm Deadpan Snarker, while Kyle was an impulsive, bold Hot-BloodedSnark Knight, especially in Cartman's presence.
Tweek and Craig fit this trope, as well. The former is hyperactive, paranoiac and anxious, who shows his emotions easily, while the latter has a darker personality, being stoic, pragmatic, monotonic, and ironic.
Blitzwing has a Split Personality represented by his three faces. The black face is The Mad Hatter, but the red and blue faces are textbook red and blue oni: the blue face is cold, calm, and calculating (and uses ice powers), while the red face is brash, impatient, and agressive (and uses a flamethrower).
Even his alternate modes reflect this; the blue face wanted to choose a fighter jet, maneuverable and good for attacking from a distance, while the red face insisted on an assault tank, heavily armored and bristling with firepower, ideal for being right in the middle of the action. After they'd argued for a bit, the crazy face stepped in and, cackling madly, scanned both; which one he uses depends on which of him is driving.
Interestingly, while you'd expect Jetfire and Jetstorm to follow the trope (they even have red and blue color schemes, respectively), they avert the trope by both acting pretty red. Of course, they are working for Sentinel Prime, so who knows how that affects their personalities.
Sentinel Prime is the red who is reckless and an arrogant jerk, Optimus Prime is blue, being far more level-headed and humble to everyone. Ironically, Sentinel's primary color is blue, while Optimus' is red.
Then there's Bumblebee, who's Hot-Blooded, and Prowl, who is more mature and more experience in fighting.
SWAT Kats: Chance 'T-Bone' Furlong and Jake 'Razor' Clawson. And when they're seen outside their vigilante and work clothes, they're wearing their respective colors.
Leonardo (blue) and Raphael (red). Reversal in that Leo is the leader of the team (aside from Splinter). Raph does the Blood Knight bit when he takes the guise of Nightwatcher in the CGI movie. It was noted with this exchange from the first episode of the '03 series:
Raph: Teacher's pet!
Leo: Ninja dropout!
Even the other two turtles, Donatello and Michelangelo, fit this trope. Michelangelo being the wild, extroverted, and more emotional Red to Donatello's calm, introverted, and more logical Blue.
Inverted with Pablo and Tyrone of The Backyardigans: Pablo's blue, hot headed, and panicked a lot in season 1, placing him as the Red Oni. Orange-furred Tyrone, on the other hand, is calmer and more level-headed, which are true aspects of a Blue Oni.
In the cartoons series for Disney's Hercules, Ares the God of war and Athena the Goddess of wisdom, who were siblings and rivals, were perfect representations of this trope. The crossover between Aladdin has Jafar and Hades under these with their colors.
Aladdin: The Series: Aladdin: red/purple and white, uses his own strength, lives with a mess of friends, has a monkey sidekick, versus Mozenrath: dark blue and black, uses magic, lives alone in a ghost city, has an eel sidekick.
In Taz-Mania the Taz/Dingo shorts, Taz plays the role of the Hot-Blooded red oni while Digeri Dingo plays the role of the cool and cunning blue oni. Also, Molly and Jake, with Molly being the easily-stressed red and Jake being the more cheerful blue.
In Bonkers, Bonkers (red) and detectives Lucky Piquel and Miranda (blue).
Gir seems to show some signs of this trope, though the colors are reversed. The first few seconds when he was active, the episode "Gir goes Crazy and Stuff", and occasionally through-out the series, Gir's colors switch from blue to red and back again. When red, he seems to show the traits of the Blue oni and is intelligent, calm, and eager to serve his master (often saying "Yes my Master!"). When Blue, he acts more like the Red Oni and is wild and over the top, with no control over his own urges or actions.
There is also the scene were Zim hypnotises Dib and the scene shows the ground falling apart with Zim in red and Dib in blue. Makes sense, since Zim is known for being impulsive and not thinking before acting, where as Dib is more scientific and logical (if only by a little bit).
Dagget (red oni) from The Angry Beavers is highly emotional and prone to Donald Duck-esque rages; stark contrast to his older brother, Norbert (blue oni), who is cool-headed and considers himself suave.
Trevor and Aeon from Ćon Flux. Subverted, in that Aeon, despite being a cool, calculating, cause-oriented Blue Oni, is meant to represent chaos and anarchy, whereas Trevor, the vain, arrogant, people-oriented Red Oni, represents order and stability.
Out of the stretchy superheroes in Batman: The Brave and the Bold, Plastic Man, who wears red, is much more blunt and okay with stealing valuables from the scene of the crime, while Elongated Man, who wears purple (it's a kind of blue) is much more analytical and noble. Plas has a similar dynamic with Batman himself.
In Titan Maximum, the original Titan Force Five had the Hot-Blooded Palmer and his Lancer Gibbs (they even wear red and blue respectively). However, the fact that Gibbs was the Only Sane Man of the team (as well as Palmer leaving him to die just so he wouldn't miss a hot date) led to Gibbs turning evil in the first episode.
The Amazing Chan and the Chan Clan has two examples, one with an inversion in color: Sweet-tempered Suzie wears blue while boisterous tomboy Anne has a lot of orange in her outfit, but responsible Henry wears orange while impulsive goofball Stanley wears blue and green.
One episode ("Alter Ego") of the 1970s Filmation Sabrina, Superwitch cartoon featured Damina, a raven-haired Sabrina lookalike with a shrill, grating voice. Depending on whom you ask, Damina is Sabrina's "magical mirror image," Sabrina's alter ego, or merely Sabrina's cousin, but what is clear is that she and Sabrina are literal examples of the trope; the rowdy and rambunctious Damina wore a red dress, and the demure and proper Sabrina wore a blue dress.
Henry is the red oni (he's energetic, idiotic (not to extreme levels)), while June is the blue oni (cool, cynical, smart). With June, it kind of makes some sense: she does have blue hair, after all. Ironically, Henry wears a blue shirt, where as June wears a red jacket.
In Life with Loopy, Loopy is the red oni (energetic, life-loving) to her older brother Larry's blue oni (inteligent, shy). Oddly enough, the colors of their hair are the opposite (Loopy has blue hair, Larry has red).
The Mooninites. While they are both still immature jerks, Ignignot is much more level-headed and sarcastic than the vulgar, in-your-face Err.
The Plutonians also follow this, with loud, dictative, and stubborn Oglethorpe, and calm, easygoing, and sensible Emory.
Among the main characters, Master Shake is definitely red; Meatwad can be red or blue depending on circumstances or pairing; and Frylock is generally blue.
Beavis And Butthead. While both of the boys are active pranksters and neither of them show good judgement, Beavis is Hot-Blooded, brash, irritable, and violent, while Butthead is more laid-back, calmer, and (barely) smarter. Ironically, Beavis wears a blue shirt while Butthead wears red shorts.
Wakfu has this dynamic between Amalia and Evangeline. Amalia is a Rebellious Princess who absconds for little reason other than boredom and brattiness. Meanwhile, Evangeline starts as a stoicTyke Bomb with Undying Loyalty to the Sadida royal family. She is all about duty and self denial and doing what's right. But as things go on, they both learn from each other and end up reaching more balanced positions on the matter of responsibility. Amalia learns from the example of Evangeline's courage and commitment, taking up the challenge of living up to her royal responsibilities and defending the Sadida kingdom at its darkest hour. Evangeline, at the same time, learns how to enjoy herself without compromising her duty.
Freakazoid and Cosgrove in Freakazoid!. In the episode "Two Against Freak", RoddyMacStew fills in as the Red Oni while Freakazoid is in the hospital. They're even - albeit unintentionally - in the right colors: Freakazoid's in red tights and Roddy's kilt is red, while Cosgrove is always in his blue police uniform.
Rogue and Jean, as the Red and Blue Oni respectively. Rogue is cynical, tough and more impulsive and willing to break the rules, while Jean is intelligent, nurturing and more levelheaded and willing to follow the rules.
Used a bit with the main six in Recess when broken into three groups of two:
T.J. as the red oni (hot-headed, easily excitable) with Vince as the blue oni (cool, sarcastic)
Spinelli as the red oni (hot-tempered, tough) with Gretchen as the blue oni (smart, laid back)
Mikey as the red oni (outgoing, drama king) with Gus as the blue oni (shy, quiet)
Legend Of The Dragon has the dragon twins Ling and Ang. You may think from looking at pictures that Ling is the blue Oni and Ang is the red Oni. However, in terms of personalities, Ling is the red Oni and Ang is the blue Oni.
Foster's Home for Imaginary Friends: Bloo (red oni) is obnoxious, self-centered, and rude, while Mac (blue oni) is intelligent, shy, and always wants to do the right thing (most of the time). Ironically, Bloo is well, blue, while Mac wears a red T-shirt.
Gwuncan, also fits for this trope, as Duncan is a red oni, while Gwen is a blue oni.
Loud, fun-loving, impulsive, bone-headed, optimistic Owen (red oni) has an Odd Friendship with the quiet, anti-social, calculating, intelligent, gloomy (blue oni) Noah. Ironically, Owen wears a white t-shirt with a large blue maple leaf on it while Noah wears a red sweater vest.
Outside of battle, red-clad Iron Man often acts more brash and short-tempered than the calm, reflective, and blue-clad Captain America.
For most of the first season, The Wasp loves the exciting and beneficial qualities of crimefighting, but her colleague, Ant-Man (whose red costume seems ironic in this case), prefers to confront criminals by talking things out and rehabilitating them. They switch roles during scientific projects, which Ant-Man enjoys more than Wasp does.
One of Wasp's friends, red-clad Ms. Marvel, confronts invaders with violence while Ms. Marvel's blue-skinned alien protector, Captain Mar-Vell, believes in finding peaceful resolutions. Their first appearance, as ordinary Army majors, contained another visual metaphor: Ms. Marvel wore a red scarf and Mar-Vell (disguised as an earthling) wore a blue suit.
The Heroes For Hire include hot-blooded Luke Cage and restrained Iron Fist. Their different temperaments get lampshaded while Iron Fist meditates.
Ozzy and Drix has this with a color inversion just like the movie it was based on, mentioned above, with Ozzy as the Red Oni and Drix as the Blue Oni. This is despite Ozzy being blue and Drix being red.
In Looney Tunes, Daffy Duck serves as the hot-tempered, overacting, obnoxious red oni to Bugs Bunny, who is a calm, cool, intelligent blue oni. Bugs himself becomes the red oni when contrasted to Cecil Turtle the the Tortoise and Hare based cartoons.
Other relationships show signs of this; for instance, Applejack is a Blue Oni to the more gung-ho and impulsive Rainbow Dash, but a Red Oni compared to the prim Rarity.
This is a list of the Mane Cast (and Spike) by order of most red to most blue: Pinkie Pie, Rainbow Dash, Applejack, Spike, Rarity, Twilight Sparkle, and Fluttershy
The Cutie Mark Crusaders count as well. Scootaloo being the Red Oni to Sweetie Belle's Blue Oni. Applebloom is generally the Blue Oni to Scootaloo, and can be either the Red Oni or Blue Oni to Sweetie Belle.
Also done with the Princesses, though played unexpectedly. Celestia, brightly colored Monarch of the Sun and the Day, is quite clearly the calm, rational, and wise blue oni while Luna, dark-colored Monarch of the Moon and the Night, is the short-tempered and emotionally driven red oni.
As far as the fanbase is concerned, Lyra is the overly excitable and emotional Red Oni to Bon Bon's grumpier and shyer Blue Oni.
Back to established canon, hyperactive party pony Pinkie Pie is the Red Pony to her quiet, rarely-emotional sister Maud and her bashful, even quieter twin sister Marble, but the Blue Pony to her other sister, Limestone, who ranges from simply cranky to flat-out volatile.
Princess Twilight is a red pony to the blue Princess Cadance. Conversely, Sunset Shimmer is a red oni relative to Twilight, both figuratively and literally in terms ofhair color.
Contrasting Sequel Antagonist example: Queen Chrysalis (Season 2 finale) is arrogant and charismatic — Red — while King Sombra (Season 3 premiere) is efficient and near-silent — Blue. They even team up (albeit just briefly) in the official comics, and the dynamic is only further reinforced.
Adventure Time: Their color scheme is backwards though: Marceline is a mischievous, loud, and careless Red Oni who has a bluish complexion (although she does wear red, which is weird since she eats red) while Princess Bubblegum is a polite, calm, and caring Blue Oni who is pink and wears a lot of pink (she is made out of bubblegum after all).
Zig-Zagged with Finn and Jake. It can be pretty hard to tell if Finn was the Hot-Blooded and headstrong Red Oni who relies on force and charges in headfirst and Jake is the wise and reasoning Blue Oni, or if Jake was the mischivous and bone-headed Red while Finn acts as a good natured and friendly Blue.
The Simpsons: Impulsive Homer and practical joke artist Bart clearly form the Red Oni half of the Simpson household to pragmatic, tradition-worshipping Marge and bookworm Lisa's Blue Oni.
King of the Hill: Dale and Bill are generally the red, Hank and Boomhauer the Blue. Though Hank is dedicated to the point of near-blindness and refuses to admit he has an anger problem …
In Regular Show, Mordecai (blue) and Margaret (red) deliberately follow this trope - both in appearance and personality. To a lesser extent, there's the relationships between Mordecai and Rigby, and the relationship between Muscle Man and High Five Ghost.
Ed, Edd 'n' Eddy: Eddy is short-tempered, has No Indoor Voice, hates following the rules, and follows his gut (Red Oni) while Edd (Double D) is willing to follow the rules, intelligent, rational, and thinks before he acts (Blue Oni).
Eds (relatively nice guys, trying to succeed via cunning rather than brute force) are blue to the Kankers' (blunt, insensitive molesters, always solving their problems by violence) red.
Tiny Toon Adventures has Buster who is the more intelligent and calm of the 2 stars as the Blue Oni, and Babs who is the more crazy and mischievous of the 2 stars as the Red Oni.
Mike, Lu & Og: Mike is the red oni to Og's blue oni, although she becomes a blue oni when contrasted with a certain green wearing island princess.
In Young Justice the main team has Superboy, Kid Flash and Ms. Martian as the Red to Aqualad, Robin and Artemis's Blue respectively.
Jackie Chan Adventures has Jade being all impulsive, insistent, and violent while Jackie is relaxed, thoughtful, and peaceful.
Johnny Test has Johnny (red) and his dog Dukey (blue). Johnny is shown to be very reckless, bratty, and more willing to abuse his sister's experiments while Dukey is more sensitive, cautious, and is more willing to warn Johnny of the consequences of his actions
Susan (red) and Mary (blue), Johnny's twin sisters. Susan is more hot tempered of the twins while Mary is more calmer but firmly strict. Ironically, Susan wears a blue shirt while Mary wears a yellow shirt in most of their screen time.
Super Noobs: Alien warriors Memnock (blue) and Zenblock (red). Zenblock is very hot tempered, impulsive, reckless, gruff, and often shouts at other peple a lot and he considers himself to be mean. Memnock on the other hand is more relaxed, sensitive, cautious, and is the most childish of the two. Ironically, Zenblock's skin color is blue while Memnock's is yellowish-green. Their different personalities often cause them to frequently clash at each other in terms of their ideals.
Futurama has Fry (blue) and Bender (red). Bender is shown to be insensitive, reckless, and inconsiderate of others while Fry is more laid back, softer spoken, and more kind of the two.
Zapp Brannigan (red) and Kif (blue). Zapp is very arrogant, egotistical, perverted, narcissistic, and is very abusive towards Kif while Kif is more softer spoken, sensitive, shy, and more passionate. Ironically, both of them wear red suits.
Hermes (red) and Zoidberg (blue). Hermes is very hard working but is very condescending, rude, spiteful toward people like Zoidberg while Zoidberg is more kind, sensitive, and childish. Ironically, Zoidberg wears a red shell.
In "T: The Terrestrial", Lrrr (red) and Jrrr (blue). Lrrr is more hot headed, loud, self centered, and more willing to invade planets and hurt humans while Jrrr is more sensitive, sweet, kind, and does not want to hurt humans. The fact that Jrrr is Lrrr's son is what connects them together to this trope and Lrrr wears a red cape while Jrrr wears a blue cape.
Mr. Bogus (red) and his best friend Tommy Anybody (blue). However, ironically enough, Bogus actually wears a pair of blue pants, while Tommy wears a pink shirt (to be completely fair would be the closest thing to red) though he does wear blue jeans.
It is also known that calm, and cool Wendy is Blue oni to Mabel's and Soos's Red.
Grenda is the Red Oni to Candy's Blue.
Family Guy: Stewie is red to Brian's blue, and Peter is red to Lois' blue. Other pairings within the family can be all over the place, but Brian is definitely the bluest overall. Among the guys, Cleveland is probably the bluest, Joe is generally blue but can get REAL RED REAL FAST AND REAL LOUDLY, Quagmire is usually red, and Peter is almost always red.
Helga Pataki from Hey Arnold! is an aggressive and hot-tempered red, her best friend Phoebe Heyerdahl is an intelligent and laid-back blue.
Square One TV inverts this trope with their animated sketch series Zook & Alison, The boy, Zook, is loud, nervous, and outgoing, but is Blue in color, while his Pink sister, Alison, is calmer and smarter.
In Sidekick! Vanna while an Alpha Bitch, usually (When she isn't pissed off at Eric) hides her emotions and acts pretty chill most of the time (Blue) As opposed to Kitty who is extremely hyper and excitable (Red) as well as Eric is the rational Only Sane Man (Blue), while Trevor is impossibly impulsive and does literally everything he possibly can without thinking ahead (Red)
Tommy and Chuckie from Rugrats are respectively Red and Blue.
Star Wars Rebels: Garazeb "Zeb" Orrelios is the Red to Agent Kallus' Blue. Zeb's the gung-ho, Proud Warrior Race Guy with a hot temper is on the side of the Rebels to Agent Kallus' calmer and more orderly, manipulative nature for the Empire. Both of them found mutual respect for one another based off their adherence to their honour code and Deadpan Snarker tendencies and the fact that Zeb saved Kallus' life when they were stranded on one of Geonosis' moons.
There's Hot-Blooded, cocky, casual Amethyst and precise, intelligent, sometimes inflexible Pearl. However, Pearl isn't as together as many a blue oni - her insecurities can lead her down the wrong path to a much larger degree than the typical red oni impulsivity of Amethyst. They're constantly arguing over how to do things, but can pull it together when they need to (if the emergency is big enough, or when leader Garnet has well and truly had it with their bickering, or, most of all, if Steven gives them that teary-eyed look.) You'd think they're as red and as blue as two characters can possibly get until we meet...
Ruby and Sapphire, the two Gems who fuse into Garnet. When forcibly separated and put into different prisons, Ruby is angry and nerve-wracked, going around screaming Sapphire's name as soon as she's freed; Sapphire, in contrast, calmly sings her in cell, and uses her freedom to quickly and efficiently hunt Ruby down so that they can fuse again. This extends to Personality Powers in "Keystone Motel", as the pair respectively become very hot and very cold - quiteliterally - when they're angry at each other. Keystone Motel also lets us get to know them better - Ruby's emotions are very much worn on her sleeve while Sapphire is outright The Spock.
The titular Steven is goofy, outgoing, and wears a red shirt, while his best friend Connie is more quiet, intellectual, and wears blue in the first intro and several episodes (for example, "Alone Together".)
In Thunderbirds Are Go Scott and Virgil Tracy are red and blue, respectively. Especially in the episodes "Crosscut" and "Recharge". Virgil also acts as the blue Oni to Gordon's red.
John acts as blue Oni to all the other Tracys (and EOS).
Samurai Jack has Jack (Blue) and the Scotsman (Red). Jack is quiet, stoic, and relaxed; the Scotsman is loud, boisterous, and passionate.
In the 2017 continuation, Ashi acts is the determined Red oni to Jack's sullen Blue.
In Transformers: Robots in Disguise, We have Sideswipe and Strongarm. Sideswipe, rebellious punk with a red paint job as the Red Oni and Strongarm, by the books rookie cop with a primarily blue paint job as the Blue Oni. In the 2001 series by the same name, there was serious, by-the-book Prowl who becomes a police car, and Side Burn, the cocky hotshot sportscar who keeps flirting with red sports cars (it's hard to know if he understands that they're not sentient... and harder to know what he thinks of his own Mid-Season Upgrade that makes him a red sports car).
Class of 3000 has Li'l D and Philly Phil. The former is more impulsive, a bit rude, and more likely to take charge, while the latter is more easygoing and more of a follower. Ironically, Li'l D has the cool color palette (green shirt and blue pants), while Phil's the one with the warm palette (yellow clothes and red hair).
Played with slightly in season 2, where Phil's a bit more emotional.
Sisters Emma and Kitty. Emma is the blue Oni as she is more cautious, intelligent, and more focused while Kitty is the red Oni as she is more fun loving, enthusiastic, and reckless.
Cadets MacArthur and Sanders. MacArthur is in the red due to her aggressive, reckless, and impulsive behavior while Sanders is in the blue as she is more cautious, is more willing to uphold the law and play fair, and tries to calm her racing partner down a lot. Ironically, they both wear blue clothing.
Jacques and Josee. Josee is in the red due to being very hot headed as she throws tantrums when she and Jacques place anywhere other than first and Jacques is in the red as he constantly fears Josee's tantrums and constantly Screams Like a Little Girl.