Dean could totally be a mentor type! Sonny got in a couple brawls and learned how to keep his own because of his father being an ex-boxer, but he was never much of a fighter before the Ghost Corps. Someone that could help him learn actual combat (other than sink-or-swim field experience) and the ins and outs of a 'military' life would be great.
Plus they could have their own Morse Code of eyebrow raises, steely looks, grunts, and nose wrinkles. The Tough-But-They're-Actually-Nice-And-Caring Guys Cipher.