Casey Ryback is a cook.
That is the absolute truth and to most people's mind, that is all he is, except for being quite open about his thoughts and his attitudes towards people, especially dirtbags and most especially towards superior officers in the U.S. Navy that happen to be dirtbags. Ryback is serving as a cook aboard the USS Missouri (BB-63) when we first meet him, preparing to say farewell to both the ship as it begins it decommissioning process and to his captain with whom he has a storied history and considerable rapport.
Of course, we know without being told going into that first of two recorded adventures that there is considerably more to Ryback than just being a Chief Petty Officer in the galley of a warship.
Ryback had been a Navy SEAL, a decorated leader of SEAL Team Four for many years. He was ousted from that highly respected organization after an altercation with his commanding officer after a poor decision by that man, based on inaccurate and highly doubtful intel, botched a mission in Panama during the invasion of that nation to take down General Noriega; said mistake leading to the deaths of several of Ryback's men. No matter the provocation, striking a senior officer is forbidden and Ryback was lucky to be allowed to remain unincarcerated and in the Navy, albeit with a new rating of Cook.
We will learn as that initial adventure ends that the black mark on his record will be expunged and he will be either promoted to or returned to the rank of Lieutenant in the SEALs, able to wear that uniform complete with the impressive array of medals including the Navy Cross, Silver Star, and the Purple Heart with clusters.
It is as a cook, a major passion of Ryback's, that we first meet him and it is as a cook that in our second encounter with the man, he has left the Navy and gotten a job as such in a cafe in Denver. It is as a former SEAL still very capable of taking the fight to the enemy on their turf and still walking away the victor.