Within recuit training platoons, the SDI is the platoon commander. The J-Hat is second to the SDI. You could think of them as Platoon Sgts. The kill hats are the 2 most junior Drill instructors regardless of rank. Kill Hats and J-Hat will interact with recruits the most and will rotate on duty and training.

For example, my SDI was a GySgt. The J-Hat was a Sgt and the 2 kill hats were a SSgt and GySgt. This isn't normal but it happens.