Sebastian Gray is a sergeant in the U.S. Army. Sean Ray is an agent with the DEA.
We learn early on that Gray had been a soldier all his adult life, signing up upon graduation from high school and serving two tours in Afghanistan followed by two tours in Iraq followed by the tour he is on in Syria helping the Free Syrian forces combat ISIL. A horrible set of events will end with him being in Corpus Christi trying to recover from very serious wounds.
Sean Ray is a "athletically built man" with "tanned, olive skin", a Texan born and raised and proud of it, who, when we first encounter him, is A DEA agent and one of five Foreign-deployed Advisory and Support Teams, or FAST. All the out-of-country assignments he had had up to when we meet him had been to Afghanistan. This trip is to Mexico where he will discover first-hand that some of the Federales with whom he must work are working both sides.
The two will meed in the first adventure when they go up against Islamic terrorists striking the facility in Texas both are visiting. Their need to work together to first survive and then take the battle to the enemy will make a very good partnership.