逆
ぎゃく · gyaku · JLPT N3 · na-adj
opposite
辶 walk + 屰 upside-down person = going the opposite way; gyaku ≈ go back
屰 is the character for a person (大) drawn upside down. The 辶 radical marks movement, so 逆 first meant meeting someone head-on as they walked toward you. That turned into opposite or reverse, as in 逆転 (gyakuten), a reversal.
逆 — 辶 walk + 屰 (inverted 大 person)
逆 combines the walking radical 辶 with 屰, a pictograph of an upside-down person. The original sense was 'to meet, go against the flow,' as if walking toward someone coming the opposite way. From this it extended to mean 'reverse, contrary, disobey.'