The army leader can be either your own PC or an NPC that you appoint as army leader. If you have mediocre stats, then it makes sense to find an NPC with better stats and make the fellow your army leader. It is important to note that your stats will be averaged with those of your army leader, so it is advisable never to use an army leader with lesser abilities than your own. Historically, the army leader would be your "advisor," as you would technically still be in charge, and expected to be out there in the thick of things.

If the NPC army leader is killed in a battle, your army will disperse. On the other hand, if the NPC army leader dies of natural causes, the army will remain together, but can only defend. This gives you time to find another NPC army leader, or take over command yourself.