Who are the defense officers in the game?

Click on officer, and then go to gallery, there will be symbols above the officers, defense will be among those.

LuBu, Mr. Figure Samurai, Lynn, Reindeer X, Captain Glory, May, Robinson, Sgt Max, Nelson, Fluffy Bear, and a few I am sure I left out. As stated in previous answer, they have a shield on the bottom left of the officer figure in gallery. The officer skills will also end with "takes effect when assigned to defense department". If you do not assign them, they won't help you.