人は見かけによらぬもの (ひとはみかけによらぬもの): appearances can be deceiving; don't judge a book by its cover

appearances can be deceiving; don't judge a book by its cover

People are not what they look like. Usually said in pleasant surprise, after someone turns out far kinder or far tougher than their manner suggested.

A Japanese proverb (ことわざ)