Due to REX being so slow, Iriyn tends to stay with the ship, never straying to far out into the fight as he prefers to be a terror from a distance. With that in mind, he usually perches himself on the top of the vessel, his mecha locking itself down with a strong magnetic current on the thickest part of the vessels hull so as not to disrupt any devices down below such as communication and radar. From here, he picks off his targets one by one. The closer they get, the easier they are for him to shoot. The rifle is a rather quick shot and the body of REX can turn fairly quickly, belying it's slow advance forward when it's flying.
Iriyn, much like his mecha, isn't a very fast individual. He prefers to out do his opponent in sheer strength. Able to take multiple hits without showing any pain or any weakening, he can usually shock his opponent into making mistakes, which will then give him the advantage of breaking them literally. Iriyn isn't one for prolonging a fight, though he does enjoy a good opponent every now and then. Iriyn is a fine shot with his hand gun, just like he is with his sniper rifle in REX. His swordsmanship isn't lacking either, though he prefers his gun to the blade. In addition to this, he has a tactical mind, and is able to make on the spot decisions that would benefit him the most, especially during a fight. He can spot enemy tactics while sitting in his mech and observing from the cockpit through the rifles long ranged scope and relay to his allies exactly what they're missing and how to deal with it accordingly. To him it is like a chess board, something he's very good at.
However, concentrated hits will put him at a disadvantage as it is still just metal. Metal breaks eventually. In the case of getting behind him, Iriyn will usually disengage himself from the vessel to face his opponent. However, this drops his maneuverability because the REX is near impossible to turn around in time to do much to an attacking mecha that is faster than he is in flight. With that in mind, he will do one of two things, take the hits as he turns to destroy the enemy fighter's machine in one powerful blow, or retreat into the vessel he's occupying. You had better hope that he simply retreats, because otherwise he's going to be very pissed off you snuck up on him.
Outside of the mecha, though he's a formidable man, he isn't invincible. He's no martial artist for sure and because he's not very fast either, he can be overwhelmed in the right situation. Manage to disarm him, and you may have a chance. He will wait for an opening, which can lead to his own injuries crippling him if they pile up too quickly or the injuries are critical to begin with.
Iriyn isn't one to like having prisoners either, though he'll do so if he's ordered to. Nine times out of ten, though, he'd rather put a bullet through the person than sit around asking questions all day. His mind works around that of a chess game. Each move he makes is meticulous and well thought out. He doesn't waste anything. If he can use someone, he will. If not, than he won't bother. Keeping his allies alive on the battlefield is only for his own preservation due to his weaknesses. He is a smart man and knows what he can and cannot do, so in order to survive he will manipulate those around him in getting him out of a sticky situation (and possibly putting them into it instead while he leaves).
In addition, Iriyn has no problems with authority, following orders to the letter. Being a Captain himself, he has no qualms about giving out orders either. He expects his command to be followed without questions and doesn't tolerate those who disagree with him openly. Iriyn know's he's not the easiest man to get along with, and thus realizes that his subordinates probably talk behind his back. Which would be wise for should he ever hear it face to face, he would find a very cold and dark place for them to wither away in.
The rest of his history is unknown and will later be revealed as the game progresses.