応 (Kanji for react)

Onyomi

How will you react when you find a human heart on the floor of the cave??? OH shit!!!!!!!!!!!!!!!!!!!!!!! is how.

一応(いちおう)

for now, for the time being, ★★★☆☆
一 (one) + 応 (react) = 一応 (for now, for the time being,)

反応(はんのう)

a reaction ★★☆☆☆ FP
反 (anti) + 応 (react) = 反応 (a reaction )

a reaction - despite the 反 kanji, 反応 is not neccesairly a bad reaction. As a noun, it's value-free. (careful! This is pronounced NOU, not OU)

応援(おうえん) する

cheeer on ★☆☆☆☆ KANA
応 (react) + 援 (assist) = 応援 (cheeer on)

to cheer for or root for someone at a game.