There are four extra models for the custom officer. To unlock them each in sequence, you need to train a new officer and achieve an attack rating in any weapon of 55 by month #4 (i.e., if your attack rating is nowhere near 55 when it says 9 months unitl trial, you should reset and try again). Once you do so, a Strike Ninja will appear and challenge the school. Defeat him, successfully finish training and save the game. These new models have higher starting stats than your default models.

Hint: To reach attack 55 easily, choose any male character mesh and look for a high (35+) attack rating with the spear. Participate in the melee exercise and use the SQUARE-SQUARE-SQUARE-TRIANGLE on novice and veteran to whack a large groupings of enemies. You should also use musou on the strike ninjas to regain life on Veteran or Expert levels of difficulty.