Start with a Private rank:
At the title screen, press Square, Circle, Triangle, then start a new game.

Start with a Sergeant rank:
At the title screen, press Triangle, Circle, Square, then start a new game.

Start with a Lieutenant rank:
At the title screen, press R1, L2, L1, R2, then start a new game.

Start with a Captain rank:
At the title screen, press Circle, R1, Circle, L1, Triangle, R2, then start a new game.

Start with a Major rank:
At the title screen, press L2, Square,R2, Triangle, L1, Circle, R1, then start a new game.

Start with a Colonel rank:
At the title screen, press Triangle, Square, Circle, Square, Triangle, Circle, then start a new game.

Start with a General rank:
At the title screen, press L1, L1, L2, L2, R1, R1, R2, R2, then start a new game.

Start with a Master of X-Squad rank:
At the title screen, press Circle, Circle, Circle, Circle, Triangle, Square, Square, Square, Square, then start a new game