What is a single word (or idiom, or metaphor) that can be used to describe a person's personality that is "hard" or "tough" on the outside, but sensitive and soft on the inside?
Something that says: "He might be hard to get along with at first, but once you get to know him more, you'll find he's really a kind and sensitive/empatic person."
Best Answer
From The Conversationist, Introvert or extrovert, normal or abnormal: the problem with personality types, by Nick Haslam (Professor of Psychology, University of Melbourne), July 30, 2014:
Another food metaphor would be prickly pear, which is tough and thorny on the outside but soft and sweet on the inside.
Related would be this variation of a well known metaphor: a sheep in wolf's clothing, obviously meaning looks frightening, but is a lamb inside.
Another related would be all bark and no bite, which can be used in a positive way, as in, for example, "Don't be afraid of your grandpa, he's all bark and no bite".