This is a list of everything I know about the NPC's in FFXI, if you have anything to add or correct, post it, but please post only if you know what your talking about ^^ thx

To get a NPC talk to Luto (a mithra in upper jeuno)

NPC armor does not affect DEF or any stats, its random each time to summon

After 18 calls you get the choice to give your NPC a helmet
Here is the list of weapon upgrades (Luto in upper jeuno has to tell you to upgrade before you can trade the weapon to your NPC)
Cesti, Bronze Knife, Xiphos, Claymore, Bronze Axe, Butterfly Axe, Bronze Zahgnal, Harpoon, Kunai, Uchigatana, Ash Staff, Ash Club
Thats the first list
Bronze Knucles, bronze dagger, bronze Sword, Two-handed sword, Brass Axe, Great Axe, Bronze Zaghnal, Bronze spear, wakazashi, tachi, Bronze hammer, ask pole

For example, if you want your NPC to use swords first wait till Luto tells you to upgrade then give NPC Xiphos, then after she says upgrade again give NPC a Bronze Sword.
Unfortunatly I can not comfirm much of this, if you have anything to add (such as a full weapon upgrade chart :-D) please post it. also if I am wrong about any of this, correct me thanks!