In Phantasy Star Portable 2 Infinity, not only can you invite the story NPCs to your party, you can also bring your friend's character and your own character.
Sonic and his friends' characters Gamma, Tikal, and Amy.
Through Friend Search, you can obtain other random players' characters and your friends' characters. Your own character will be shared to others as well.
Furthermore, as Terada has stated, when one invites their own character to the party, that character's level stays the same. So you can have a much easier time if you take your own character that is stronger than the one you are currently playing.
(For example, If you are currently playing as a level 1 character, you can invite a level 199 character. But that invited character's level stays the same. Since I am Ricardo (Lv90), I can invite my other character Jay (Lv100) or even Champaigne (Lvl 200).
Of course there will be more partner characters to expect other than Nagisa… dot dot dot…
They complain about it here, Gamefaqs, and Psoworld, those darn NPCs keep breaking chains, what the heck is wrong with them? Sakai has a solution! Through a shortcut command called "Follow Shift," the NPCs will no longer break chains. They will mainly use normal attacks. This will be effective against larger sized enemies.
Normally I use the hippo Mysynth in missions and never had a problem with traps. However things were different when my friend gave me the waiter Mysynth. I brought that NPC to the Caverns of Moatoob, but he had one major flaw, any time there was a turret trap, he would run up to it without attacking and constantly receive damage. Then he eventually died, revived, and constantly ran right into the turret again. Then he used his SUV against the turret, but for some strange reason the turret never received any damage. The waiter died immediately after.
In regards to traps that can easily damage NPCs, Sakai says that he has increased the endurance for an NPC to withstand against a trap. So partner characters will no longer be "imbeciles" when actively engaging a trap. So you shouldn't see them dieing as much as before. We'll see Sakai… We'll see…
[via pspo2iblog]
Can you find Kunoh?
Additional Comments
Increasing the NPC's resistance to traps doesn't seem to me to do much of anything. The problem is with the AI itself. I notice the behavior occurs with the Waiter PM and Magashi. Magashi runs up to turrets and goes all wonky, then whips out his little wand to spam Resta. This "bug" doesn't seem to occur to every NPC though. For example, in Story Mode with Yuto, Lumia, and Emilia in PSO forest, they actively attack and destroy turrets by themselves.
I don't have all the time to go through which MySynths are trap competent, but I know the AI can handle them already. You have to wonder why this programming wasn't applied to Magashi…