expression
does not match one's appearance; looks can be deceiving
Used to say that someone or something is different from what their appearance suggests. Often implies a positive surprise, e.g., a tough-looking person is actually kind. The related form 見かけによらず is also common.
See also: 見かけによらず
彼は見かけによらない優しい人だ。
He is a kind person who doesn't match his appearance.
この料理は見かけによらないおいしさだ。
This dish is tastier than it looks.
Almost identical in meaning, but 見かけによらず is more common as a set phrase. 見かけによらない is the negative form of 見かけによる and can be used attributively.
From 見かけ (appearance) + に (particle) + よらない (negative form of よる, 'to depend on'). Literally 'does not depend on appearance'.