The Japanese use English in many ways in their daily lives, for expressing words related to automobiles or technology, for example, or for decorative phrases like "image up" (to improve one's image) or "guts pose" (to strike a self-confident pose). Since English words can be unwieldy in syllable-based Japanese -- the word "weekend" takes seven syllables to expresses in katakana for example -- they are often shortened and simplified to make them easier for them to work with. Screwdrivers come in "Phillips" and "regular" but the Japanese refer to them as "plus" and "minus," which is genius. A ménage-à-trois also get a very simple name in Japanese: 3P, very logical. Often the Japanese will use abbreviations which may make little sense to native English speakers, like HP for homepage, CM for TV commercial, OL for "office lady" (a female office worker), OB for "old boy" (an alumnus of a school), NG for "no good" (a blooper made while filming, or used as the opposite of OK), or KY (short for kuuki yomenai, lit. "cannot read the air," describing someone with poor social skills).
Index is a character who is KY, out of sync with everyone around her.