Red Oni and Blue Oni is a representation of two individuals (or occasionally, factions or civilizations) that are significantly linked together somehow (such as through a rivalry, friendship, being 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.
Red Oni
Red Oni is associated with passion, wildness, and defiance. A Red Oni character is often more brawny than brainy, extroverted, enthusiastic, determined, and filled with a zest for life. They are also much more likely to break conventions and rules than their counterpart. If a Red Oni is indeed noticeably intelligent, they will still be much more wild and impetuous than the Blue Oni and/or with less sophisticated thoughts, feelings, or goals. 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 closest friends and you've just guaranteed yourself a world of pain. When a goal-oriented villain, the Red Oni will often be crazy and psychotically willing to sacrifice any innocent lives to achieve their goals. A more evil Red Oni tends to be a Blood Knight. In terms of character alignment, the Red Oni tends towards the chaotic alignments, though this isn't always the case. Blue Oni, while usually stoic and thoughtful, may hide a volatile, angsty nature that may erupt if pushed too far. In such instances the two may switch roles, with the Red Oni becoming the reassuring, composed partner
Blue Oni
Blue Oni is associated with serenity, control and observing 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 The Stoic, The Quiet One, Aloof and Snarker. 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 willing to sacrifice anything (and often anyone) for their goals, mostly only when their villainous goals are laden with true purpose. Being more calculating, conniving and thorough, Blue Oni tend to be more successful villains long-term than Red Oni. In terms of character alignment, the Blue Oni tends towards the Lawful alignments though this isn't always the case. Blue Oni, while usually stoic and thoughtful, may hide a volatile, angsty nature that may erupt if pushed too far. In such instances the two may switch roles, with the Red Oni becoming the reassuring, composed partner