Below is a brief breakdown on how to unlock each pilot. It's all very simple, tied directly to the level of influence of the Army of Chaos. Progressing through the game, liberating different regions, destroying enemy equipment, it all goes toward your Army of Chaos level.

Unlocked through story progression.
Unlocked through story progression.
Reach Army of Chaos Level 4.
Reach Army of Chaos Level 6.
Reach Army of Chaos Level 9.
Reach Army of Chaos Level 11.
Reach Army of Chaos Level 14.

That's How To Unlock More Pilots In Just Cause 4. You can also secure factories scattered throughout the game world to increase Production Speed, which reduces the time it takes for your pilots to refuel - making them accessible quicker.