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:
Muhammad Ali proposed a fruit and nut-based typology, classifying people as pomegranates (hard on the outside and inside), walnuts (hard on the outside, soft on the inside), prunes (soft outside, hard inside) and grapes (soft inside and out).
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".