反 character

Its core senses include opposition, reversal, and turning back. Mandarin uses it in 反對 (oppose), 反而 (instead, contrary to expectation), and 反省 (reflect on oneself). Translate the whole word: “anti” does not fit all three.

Chinese and Japanese share the character, not interchangeable vocabulary. In Japanese, 反転 means reversal or flipping. Recognising this reverse kanji—a character used in Japanese writing—gives you a clue, not the complete translation.

Does 反 mean someone disagrees with me?

Not necessarily: a lone 反 leaves too much unsaid. Ask for the full phrase before responding as though you have been contradicted. Objection to a proposal is not automatically rejection of you.

Its written form evolved in ancient China

Ancestors of 反 appear in writing from China's Shang dynasty, followed by Western Zhou bronze inscriptions and Warring States manuscripts. These successive periods document an evolving written character, not a modern badge designed to mean “against.”

Traditional character analysis interprets its shape as a hand turning over. That explains a way of understanding the drawing, not necessarily its original meaning. The related senses of turning, returning, and opposing do not by themselves tell you which came first.