Red Oni, Blue Oni/Fan Works

In the Great Chicken Miasma's Mario fanfic Better Off Alone, the young Yoshi Waffles is the Red to everyone else in the group's Blue.
In kirbystarwarior's 'Disgaea: Jewel of the Gods' fanfic, Laharl is the red oni to Adell and Mao's blue oni.
- Adell can also, arguably, qualify as the red oni to Mao's blue oni.
Winter War gives us Ikkaku and Nanao, the co-leaders of the Hueco Mundo mission. Ikkaku is a Hot-Blooded Blood Knight with a newly acquired habit of giving nicknames and a physical fighter with no aptitude for kidou. Nanao is a serious, methodical, control obsessive Badass Bookworm and kidou specialist who so far has avoided hand-to-hand combat.
Eri's Game has Eri as the Red Oni and her best friend, Shiki, as the Blue Oni.
Calvin and Hobbes The Series has Dr. Brainstorm as the red oni, with his Foil Jack being the blue.
To Kill a Thief has Light Yagami as red due to being crazy with Knight Templar ideas, while his partner, Miyako Toudaiji, is blue due to being sane with By-The-Book Cop tendencies.
In White Rain, it is Sasuke who is the Red Oni to Naruto's Blue, strangely enough. Their relationships also have the same distinction.