Provenance — where it came from
The kanji 陽 (yō) and 陰 (in) are the yang and yin of Chinese philosophy: sunlight and shadow, the two poles of everything. Japanese has long used them in ordinary words — 陽気 (yōki, cheerful) and 陰気 (inki, gloomy).
In the 2010s, school and internet slang fused each with キャラ (kyara, “character” — one’s role in the social cast) and produced a two-bin sorting system for humanity. The 陽キャ (yōkya) is the sun-type: loud at lunch, center of the group photo, at ease at parties. The 陰キャ (inkya) is the shade-type: quiet, indoors, happier online. Every classroom, the words imply, is already divided — the slang just printed the labels.
What saves the pair from cruelty is who uses them. 陰キャ is mostly self-applied, worn online with the same dark comfort as ぼっち (№ 035) — “well, I’m an inkya, so.” The sorting is harsh; the tone is not entirely. Two of the oldest characters in East Asian thought, which once divided the cosmos, now divide a classroom at lunch hour. The cosmos, presumably, was easier.
Field notes — how it’s used
俺は陰キャだから文化祭は裏方でいい
ore wa inkya da kara bunkasai wa urakata de ii
“I’m an inkya, so I’ll just do backstage work for the school festival.” — self-classification, then peace.