Beyond the End: Uriel and Michael. Uriel is defiant and wild, doing whatever he likes. He's known for being destructive, foul-mouthed and mannered. Michael is the one upholding all the rules, cold-hearted and unyielding. Yet the true and the closest they could be.
Bricktown: Jill is the red to Greg's blue. Greg is an unabashed nerd with a seemingly general mellow attitude, who can quickly turn into a Grade-A Cloudcuckoolander at the drop of the hat; Jill is a rabid sports fan with only hints of being a closet nerd, is generally more uptight and by-the-book than Greg, and has a Hair-Trigger Temper. To drive the point home, Greg's typical wardrobe consists of various shades of blue, while Jill's consists of shades of red.
The fan-dubbed "boat moms" of Castoff, Arianna and Marina:
Arianna is Red, and Red Is Violent: a choleric, blunt, somewhat paranoid Action Girl with No Social Skills, who mainly uses offensive magic and swears. Visually speaking, she wears a red shirt, her magi gem (thus, her magic) is red, she's tall, thin and her hair is long, straight, almost sharp.
The Continentals: A steampunk murder, mystery, sci-fi, adventure webcomic set in post Jack the Ripper England where Continental Operative Jeffrey Tiffen Smythe is the sophisticated, elegant, intellectual gentleman and his gender bending partner, the adventuress Lady Fiona Fiziwigg, is a hot tempered, foul mouthed, do and say as I damned well please malcontent and proud of it. It's a pairing of mismatched personalities that works surprisingly well as the two investigate a series of brutal "mangling" murders, uncovering a tangled web of intrigue, adventure—and murder!!! Find it here.
Elf Blood has the KO Twins: SKO is the Blue Oni, being an adherent of law, order, and obedience, whereas TKO is the Red Oni, being the runaway renegade with a passion for freedom. Judging from a photograph taken when they were in school, however, this dichotomy may have been inverted in the past.
In Girl Genius, we have the upbeat, charismatic Gil and the dour, calculating Tarvek. An interesting example in that they're both Sparks, both schemers by admission, and both part of a Love Triangle with Agatha.
In The Glass Scientists, vibrant and extroverted Hyde is, quite obviously, Red, while Jekyll, the very image of British politeness, is the Blue.
Alma and her flatmate Edith are Red and Blue respectively - one's ever-excited Genki Girl who takes photos of her desks and speaks with hearts, while the other is The Stoic to such a great extent that one may start to wonder whether she's not an Emotionless Girl.
Two Blacksmiths watching Edith. The girl is calm Deadpan Snarker, while her coworker is a fine example of Leeroy Jenkins.
Tedd and Susan, sort of. In this case, it's more of a personality difference as Tedd is generally upbeat and unobservant while Susan is a Deadpan Snarker. Other characters lean towards both sides, but generally hover in the middle.
Another notable example is Susan and Diane. A strange case, because they haven't actually met, but they're actually separated-at-birth identical twins who share a few traits but are generally Different as Night and Day. For bonus points, Susan wears a lot of blue and dyed her hair dark blue, whereas Diane's wardrobe is more red and she has naturally blonde hair.
Gunnerkrigg Court has an interesting example, where we were introduced to a red-haired Emotionless Girl (Annie) who befriends an impulsive gung-ho girl (Kat). Mutual Character Development, however, has by now reversed the roles almost completely.
Dave Strider is the Blue Oni to John Egbert's Red Oni, though their associated colors are kind of inverted. (Dave is Red/fire, John is Blue/air.) By the end of Act 5, though, they've pretty much switched places.
Karkat Vantas (red) and Jack Noir (blue) also have traces of this during their team-up in Act V, minus color correlationsnote because trolls think fashion is stupid and Jack Noir is big on black. Karkat is impulsive, loud, and almost always angry at something, while Jack is cool, quiet, and moves with a chilling economy of effort.
Karkat, again, plays Red Oni to John's Blue once they start interacting. Their colour correlations match up as well, and Karkat's first conversation (from his perspective) with John is a textbook example - Karkat is a raging, capslocking, furious, hateful mess, while John responds with a cheerily oblivious "hi karkat!".
Also, cheerful, perky Jade (Red) to grim, dour Rose (Blue). Rose ends up playing Blue Oni to a lot of people, really; she's just about as blue as it is humanly possible to be.
Equius Zahhak, a stoic traditionalist, plays Blue Oni to the Red Oni antics of Nepeta Leijon, a genki roleplayer.
Sollux Captor, due to his bipolar mood swings, plays this trope with himself. Later he mellows out and starts playing the blue to Karkat's red.
I'm the Grim Reaper: Scarlet is predictably the Red Oni to Chase's Blue Oni. Scarlet makes more decisions based off emotion than Chase does and is by far the more dangerous of the two. Chase is more reserved, logical, and introverted, although the two switch places once when Chase loses his temper interrogating Ana and reaches for his gun.
Juathuur: Dejoru is dark-haired, calm, and compassionate, with the talent to become a healer. Ratheel is loud, fun-loving, has red hair, and fights by making stuff explode. Partly subverted as Dejoru is capable of releasing an enormous amount of raw energy. Ratheel, on the other hand, is completely useless in each and every fight we see.
Once Arrio joins the Jupiter-Men, he quickly has this dynamic with Nathan. Whereas Arrio is emotional and laidback, Nathan is stoic and professional. Arrio is hot-headed in stressful situations but kind and understanding to his friends. Nathan tends to keep a cool head but is so devoted to his role as a Star Guardian that he tends to disregard the personal circumstances of others. Their personal lives are also opposites: Arrio is a Former Teen Rebel whose family lives modestly enough that he works part-time jobs and tries to cut his own sister's hair to save money. Nathan is a Lonely Rich Kid who has spent his life studying and training to be a Star Guardian and is set to inherit a billion-dollar tech company.
The Little Trashmaid: Ricky and Spencer, respectively, and it’s reflected in the colours they wear. Ricky tends to have a quicker temper, particularly towards Tidy’s cluelessness of human life, while Spencer remains calm and keeps Ricky in check.
In Next Town Over, John Henry Hunter is a pyrokinetic outlaw with a suave demeanor and a zest for life, while Vane Black is a stoic, melancholic woman who eschews all pleasure and relations in her quest to kill Hunter.
The Bobcats in The Oatmeal comics are identical in personality as grown cats... as kittens, however, Beige Bob was a physically violent bully (Red), while Grey Bob was a soft-spoken Manipulative Bastard (Blue).
Xykon and Redcloak. The former is an Faux Affably EvilSorcerer who revels in his own Chaotic Evil nature, whereas the latter is a Well-Intentioned Extremist who wants to bring prosperity to his race and whose alliance to Xykon is more a strategical move than anything else.
The Lawyers: Phil's red to Mr. Jones' blue. There is something of an inverted lampshade hung on this; Phil wears a blue tie and Mr. Jones wears a red tie.
In Plume, Vesper and Corrick are Red and Blue respectively, with the former a buddying Blood Knight girl wanting an adventure!, and the latter a Seen It AllServile Snarker. The latter even has a blue coat on all the time.
Gunhild and Sigrun are sisters from Realmwalker. Gunhild is an introverted, sarcastic bookworm who likes death metal dark clothes, and Sigrun is a bubbly extrovert who likes bubble tea, cute hairstyles,anything pink and fluffy And battleaxes.
Arguably, Sabrina and Zig Zag from Sabrina Online, the former being a somewhat reserved computer/Transformers nut and the latter an outgoing porn star legend.
Attley and Fletch, the main characters of The Sanity Circus, respectively form an oni duo. Attley is far more relaxed and active, while Fletch is direct, distant and rushes into things far less readily.
In The Silver Eye Enel is the red oni to Apen Shephard's blue oni, with Enel being the more laid-back and emotionally driven one, and Apen being the more serious and emotionally repressive one.
Sluggy Freelance has Oasis and Kusari, two gymnastic assassins of mysterious origin with superhuman powers. Oasis is basically insane as a result of Mind Control gone bad, which involved manipulating her emotional states. She is left impulsive and compulsive about the emotional reactions she was ordered to have before her controller was killed and is often incapable of rational thought. It's also dramatically revealed late on that she's pyrokinetic, adding to her fiery repertoire. Kusari obeys orders unfailingly, possibly also mind-controlled, but is still capable of rational thought and always remains cool. The two look about identical under their respective outfits considering that Kusari is always masked, but Oasis's most noticeable feature is her fiery red hair, whereas Kusari is always shown in the same bluish-purple outfit. Before their origins were revealed, there was speculation that the contrast between the two hid symbolism hinting at their unknown origins. The real explanation for their different temperaments is pretty simple in that both are Artificial Humans in some sense, and Kusari's creators, trying to imitate Oasis without knowing how she was created, didn't manage to give her a proper emotional repertoire.
Soil That Binds Us' protagonists are collected Petunia and easily-agitated Dandelion. Used in-universe, as it's shown that only Petunia can calm down Dandelion from his supernatural outbursts.
Stand Still, Stay Silent has two pairs. One consists of brash, extroverted fighter Sigrun and quiet, stoic medic Mikkel. The other consists of arrogant Pretty Boy and professional Pyromaniac Emil and quiet, strangely cat-like scout/mage Lalli.
In Sticky Dilly Buns, Ruby Larose is (despite her name) basically a lone blue oni in a universe of reds, leading to at least two rough-and-ready, low-key instances of this trope:
She and Dillon are the most obvious case, especially early on in the comic's run. Dillon, the red oni, is a flamboyant Camp Gay man, and is certainly an extrovert with a lust for life; Ruby is a virginal borderline Sour Prude, and is certainly concerned with control and observance of authority, and is also more intellectual, proud, traditional, and definitely introverted, and is a mystery at least to her sister Amber, although she is only intermittently respected. They end up learning quite a bit from each other.
The pattern recurs when Ruby recruits Zii to help her set up a commercial Yaoi fan club, as Ruby has apparently unconsciously recognized that the complementary pairing makes a good business team; red oni Zii has the life experience to judge what will work and useful social connections; blue oni Ruby has the focus and organization to carry through on ideas.
Sunbird: Salva and Andraz. They often follow the colour schemes in pinup art, though rarely in the comic itself.
Nina is Red Oni to Deirdre's Blue Oni in Threading.
Quant, the Fiery Redhead with anger management issues and an egothe size of the Tower. He is too proud to ever openly admit a mistake of his and thus comes up with pretexts when it comes to doing the things he secretly wants to. Like staying with Lero-Ro, his longtime fellow Ranker who once climbed the Tower with him. He is reserved, calm, quiet and patient, unless you start bugging him to much. Unlike the rather Book Dumb Quant he spends a lot of time thinking, especially when he notices that something is going wrong on the Second Floor, which he can't ignore due to his righteous nature (Quant is more of the self-righteous kind). He is also rather sophisticated and a passionate collector of gramophones.
KhunAgueroAgnes and RakWraithraiser, Bam's closest friends in the Tower. The former is a scheming, cheating chessmaster with a lot of brains and some trust issues, the latter a giant bipedal alligator who just wants to fight, plain and simple, which describes his world view and how he stomps through life, though, unlike Khun, he never lies (though he is a bit of a firendship-type Tsundere for Bam). Khun's color scheme is blue, Rak's is red.
Excluding Teddy, the main group can be split neatly between those who are red and those who are blue: on one side you have the boisterous Ben, trigger-happy Alex, and loud-mouthed Rowan; on the other you have the quiet Gerard, stoic Gray, and timid Eugene. This dynamic is most clear between Rowan and Eugene, who knew each other in middle school and thus spend a lot of time together.
The Mok-ha Duo are a pair of powerful fighters who contrast each other in every way possible. Dongha is an excitable lunatic who picks fights for fun, while Seongmok is serious, barely speaks at all, and only fights those that he thinks are worthy.
Many examples in The Word Weary, the most obvious being Grace and her room mate, Gary. This is especially evident after she dyes her hair red while Gary's is bright blue. Grace is angry, cynical and open about it, Gary seems calmer and tries to find the humor in bad situations, noticeably after Gary and his boyfriend break up. Other examples include John and Sam vs. Jason and Yorick, and Elly vs. Harry.
In Yokoka's Quest, Yang and Yin respectively. Yang is lively, restless, happy to interact with his customers, and has a red horn and eyes. Yin is lazy, calm, likes to plan things out, and has blue horns and eyes.
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Chaotic Evil nature, whereas the latter is a Well-Intentioned Extremist who wants to bring prosperity to his race and whose alliance to Xykon is more a strategical move than anything else.
