Two individuals who are significantly linked together (as friends, siblings, rivals, etc.) and have contrasting personalities and approaches to the world.
The Red Oni is associated with passion, wildness, and defiance. A red oni character is extroverted, enthusiastic, determined, and filled with a zest for life. The Blue Oni is associated with serenity, control and observance of authority. A blue oni character is proud, traditional, and introverted.
While many expressions of this trope play it straight, it is also not uncommon for the Blue Oni and Red Oni to mix traits.