irascible

a word used to describe a person (often a coach) who, on the surface, is easily provoked to anger or very irritable but who everyone knows, deep down inside, has a heart of gold and a warm glow of support, appreciation, and kindness. The phrase is often used to describe Michael "Mickey" Goldmill, who was a fictional character created by Sylvester Stallone, portrayed by Burgess Meredith in the Rocky film series, and who, while often very irritable had a heart of gold and always wanted the best for the kids.

That coast is irascible, tough on the outside, warm on the inside.