I got around to playing this the over the last week. After speedrunning through the game once, I successfully revived Nei last night with the aid of the gamefaqs guide (
http://www.gamefaqs.com/ps2/916725-sega ... faqs/47914) and a very useful excert from a post by UniqueUserName:
When you get Teim, talk to everyone in every town and examine all "destroyed buildings" in her home town. Also, repeat this process after she leaves your party.
Examine all bodies found on the floor (her kidnappers), there is more than one.
At Roron Dump talk to all Motavians several times using Consult before and after each one, and after receiving the Scooter but before touching it talk to the ALL one more time, including the ones in the North section of Roron.
Don't forget to pay for the Polymetryl twice, by giving it to the wrong Motavian first and getting the "no backsies" message and see the few different messages of the guy that sells it to you.
Also, some NPC's dialog changes after certain events talk to every person in every town after something major happens. Especially Phllip and Nanette's Grandfather... there was a few messages that only appeared at weird intervals... like before and after you check the broken bridge you get two different kinds of "no no" messages before you get sent to Kueris.
Before going to meet NeiFirst make one last town run and talk with everyone with every character you have, in your home, two by two. That's how I got the necklace finally... and Phillip's further weather reports after that lol.
I hope one of those was a forehead slapper for you and you get it this time.
Good Luck!
I made some saves up along the way in an attempt to verify some of the information out there but honestly the process is so grueling I was pretty burnt after testing just a few things.
I can say for certain that just completing each party member's 'quests', talking to the Governor before/after picking up a new character, backtracking to talk to everyone after the major events and being "pretty sure" you've seen everything is not enough. It takes some real diligence in regards to spamming L1 and going back and talking to everyone after every minor event until you want to throw yourself out of the nearest window.
The only thing I discovered (mainly due to it being pretty easy to test) which hasn't been mentioned before as far as I have seen is that you don't have to beat Neifirst with Nei. If you've met all the requirements you can let her fall to her nemesis and kill the cheeky wench with Eusis/Rolf instead. I know that's not exactly the most useful information, but hey it's better than nothing right?!
I'd highly suggest to anyone endeavouring to do it right the first time around to concentrate and not cut corners - you really do just want to aim to see every line of dialogue out there. Back-up saves are great but really they are only useful if you know you've messed something up like missing a time sensitive trigger(like Anne/Amy's Trimate scene) since if you get back to Paseo after beating down Neifirst and the cloning jerk tells you your lovely biomonster companion's cells have seen better days, you're probably going to have no idea where you went wrong and will pretty much have to start over anyway. Perhaps the revival trigger is based on something weird like just hitting a predesignated (extremely high!) percentage/ratio of unique text messages seen along with some mandatory ones like the 'quests'.