反 is built from the 厂 (cliff) and 又 components.
It belongs to a phonetic family with 販, 版, 坂, and 板 — they share the 反 component and the reading ハン.
A common word using 反 is 反対 (はんたい), meaning “oppose, opposition, resistance”.
Don't confuse it with 坂, 板, 版, and 販.