This is the 2368th most frequent Japanese word.
ク
The Japanese word "ク" (ku) is a katakana character, primarily used for foreign loanwords, onomatopoeia, or emphasis.
In this sentence, 'ク!' is used as an exclamation expressing a reaction to spicy food.
彼は辛い食べ物を見ると、顔をしかめて『ク!』と言う。
When he sees spicy food, he grimaces and says 'Ku!'
Here, 'ク' refers to a button labeled as 'Ku' on a game controller.
このゲームは『ク』を押すと、キャラクターがジャンプします。
In this game, pressing 'Ku' makes the character jump.
In this context, 'ク' functions as a nickname for an individual.