So far, they attack after you make the first swing upon the target.
If you enter /healing, your NPC will do the same.
The NPC's starting mode is "Shield"... occasionaly, at the start of battle, they will use the Job Ability "Provoke." Also, occasionaly, they will use a Weaponskill (Mine has used Burning Blade and Fast Blade)
Starting job level of the NPC is set to 30.
I do not know if they get aggro like the NPC quests do.
You CAN cast cure on other's NPC's in your party. You may also target them with -na's and erase. You may NOT use Protect spells on them. Bard songs have no effect on them.
They are quite strong in taking hits, even as a taru, mine takes hits much better than me. >_>
~That's all I've figured out about them. Also, mine upgraded to Lizard armor, she's becoming less gimpey :P (About 2 NPC call's done so far)